Basically the engine & frame are as per the B40 Rough Rider .
As Shane has mentioned there are parts books for the GA but it is just that , a list, no exploded diagrams so you will need the Rough Rider parts book for sequences.
The big end will be buggered.
The army specified B50 needle rollers which would have been fine if they did not combine it with the C15 trials 1st, 2nd & 3rd with the std BSA 1:1 top.
Thus the bike got laboured in top because the jump 3 to 4 is a valve bounce to idle job. Downchanging 4 to 3 can and does lock up the the wheel despite being 6.5:1 compression
Labouring in top allowed the cage to skuf on the outer race eventually breaking up . I have a box full of rods with the cage remenets hammer welded to the outer race.
If you were lucky it fell out leaving you with a not so crowded ; crowded race and the bike will actually run like that because it is very low compression & the piston can not hit the valves.
I rode one of mine liken that for years in my impoverished youth.
If you drop a B44 box in you can run the B50 bearing , otherwise go for a Std B40 / B44 big end.
Get a copy of Rupet Ratio. there are 3 different length big end journals. You can fit any one but you have to space out the crank accordingly
Other little differences is the GA's are 6 V with the 66 multi pin ignition & lighting switches.
I geared min up as high as I could on the gearboox sprocket and ran it as a 3 speed bike leaving top as an overdive for long flattish roads like expressways.
It was fitted with the ET points cam ( always open, close open then fire ) and was designed to run battery less .
Oil filter fitted under battery.
Got lots of photos if you need then for reference .
The fully enclosed chain guards are very hard to find as most tossed the lower & rear sections.
Single leather pannier on left side, despatch box on the right, 2" thick hardwood duel seat and high rise right hand muffler ( also impossible to find ) as it is an empty case over a perforated pipe similar to Brooklands style mufflers.
Long neck oil tank that eats your right thigh when kick starting.
626 carb with long spacer that has a drain as the carb is mounted slightly cantered forward. The carb is jetted to allow for the air hole between it & the inlet valve but the drain tube must be 18" long exactly.
Air filter is tool box which I think is also unique but a std pillbox element can be made to fit.
There is no record of the actual rego numbers and like a fool I washed mine the instant I got it home thus removing the dirt outline of the rego number.
There were 3 finishes
1) Olive drab, full viterous enamel
2) RAAF blue, also full vitreous enamel
3) MP , Olive drab with chrome handlebar levers & wingscreen
On all others all normally chromed parts were painted dull silverfrost
I have the only bike that was for sale out of the 280 sold from Moorebank in the final 2 auctions that had divisional markings ( signals 40).
All others simply had the std AIF kangaroo stencelled in yellow on the right side of the tank and NO BSA on the bike anywhere other than the timing cover in the primary . No MADE IN ENGLAND's or any other markings apart from the PROV_UR or CERT_UR stencilled on the tanks showing they were Uneconomic Repairs.
There were at least 3 C15's that were used during evaluation and remained in service .
Large tank which is identical to one of the Gold star tanks that did not have any badges fitted & for that reason are now unobtanium as they all got sandblasted chromed and suddenly increased in value by a factor ot 10 as genuine GS parts.