Thanks for your interest!
In no particular order of priority...
My intention was to base this whole extended concept in what *might* have been likely around 1920 if history had been nudged in a slightly different direction and if, post-WW1, Britain had been more inclined to undertake what we would these days describe as a "strategic defence review" with a subsequent "operational re-structuring" of her armed forces.
Equally, although I'm trying to take into account established service traditions and the prevailing social and political circumstances of that time, I guess I'm also, perhaps, assuming a slightly more modern and open-minded (and less hidebound!) approach to learning the strategic, tactical, political and leadership lessons of WW1...!
In that respect, I'm not trying to match the grades to those which would fit with the contemporary structures of the British Armed Forces, or even those of the *real* 20th Century history. My plan with the RAF in particular was kind of to go right back and say, OK, if *I* was starting from scratch, founding a whole new branch of the armed services, what might *I* have come up with, given the limitations I had set myself.
To reply to your comments specificlly:
I know that the rank of Corporal in the RAF was established along with Sergeant, etc. from the RFC but my deliberate intention here was to remove the anomaly of the very different variations in grading relating to the term "Leading..." between the RN and the RAF; it seemed to make sense that a "Leading Aircraftman" in the RAF should be directly equivalent in rank to a "Leading Seaman" in the RN. I chose this over the equivalent Army rank of Corporal because of the new nature of this fighting force where the majority of personnel were not direct combatants but were specialist support staff whose role was to enable the activities of a smaller number of combat pilots and other aircrew. I made the exception for the rather more overtly traditional military roles in the RAF Regiment and the provost role in the RAF Police. This didn't seem unreasonable, given that the Army clearly already used the two equivalent titles of Corporal and Bombardier at the same rank.
I fully take the point that Lance-Corporal was, for a long time, a local regimental appointment and not a substantive rank in its own right; I guess this was one of the things that I was thinking of bringing forward in the historical timeline in order to achieve a more sensible banding of equivalent seniority and responsibility across the different services. With that in mind, it seemed reasonable to establish Able Rate and L/Cpl as broadly equivalent on the first tier of supervisory responsibility.
One may argue legitimately, however, that an able hand is really more equivalent to a fully trained and experienced Private (or Gunner or Rifleman or Guardsman or Marine, or whatever) and I wouldn't necessarily disagree with you. Maybe what I actually need is some sort of "Senior Rate" and "Senior Aircraftman" (exact titles to be determined) to take on the Lance-Corporal role and then have "Able Rate" and "Aircraftman, First Class" as the "fully qualified" basic role, thus making "Ordinary Rate" and "Aircraftman, Basic" the grades for those who have not yet completed operational training. Junior Rate and Aircraftman Recruit (or something like this...) would then be the official titles for the raw newbies.)
I would still postulate "Marine" as the basic title that replaces "Private" for *all* unranked enlisted RM, although, again, this could technically be denoted "Marine Recruit" for those in basic training. Something similar would probably work for Army raw recruits until they pass out of basic training and are posted to their respective regiments, at which time they gain whatever title their Regiment uses for unranked soldiers.
Overall, however, my inclination would be not to differentiate formal titles between phases of basic training — they're either fully qualified, competent and on-strength or they're not. There may well be a case for some differentiation to be made within the recruit grade, especially if they're moving up to some sort of hands-on operational training, but I would keep it simple and say "fully-qualified" or "not-qualified" and not specify "part-qualified"
Back to the RAF for a moment, and I just wanted to point out that my use of the progression of different numbers of blades on the propeller insignia for the technical trades was an attempt to offset the situation where it is necessary to acknowledge different stages if technical training without it being necessary (or appropriate) to appoint someone to the next substantive rank. It is clearly possible to achieve technical competence without needing to then directly take on the next overtly supervisory role, which is what happens if you can only promote within too rigid a disciplined structure. (Logan, I guess this also partly explains why the RAF ranks evolved as they did through the 20th Century — one other option being the US approach of having rather a lot of poorly differentiated enlisted grades all hovering around as middle-ranking NCOs...)
With regard to things like specific insignia variations for certain appointments, yes, absolutely: I still expect H Div RSMs to have the traditionally huge Tate & Lyle adverts on their sleeves but equally other roles, such as the Master Gunners RA, would also retain their special appointment badges too (I just haven't drawn them yet...!).
Oh and I almost forgot: the RM side caps were a bit of a new creation — although I'm not sure how close to reality I may have come; I'll have to research this one a bit more. I've always quite liked the idea of side-caps in regimental colours so I figured that the amalgamation would be a good time to replace the older-style RM caps and bring them more into line with the Army, just in RM colours. The small differentiation between officers and other ranks just kinda looked good when I tried it — so I kept it...! The red piping on navy side caps for other ranks reflects the scarlet crown piping on their SD peaked caps (OR caps only in this history) whereas the full scarlet crown in the officer version reflects the red hatbands on their caps. Senior ranks then add gold piping to reflect the oakleaves on their caps.
I'll have a re-think about how I want the junior enlisted ranks to work and how the different grades and titles might match up across the four distinct services. I won't have much time this week but I'll get back to you when I think I have something that works for me.