Keith;
The Arm of Service Marking: white '6' on a red over blue square is definitely for a Light Anti-Aircraft Regiment. But whether it's for the 49th L.A.A. Regiment, R.A. or for the 121st L.A.A. Regiment, R.A., 6th A.A. Division, I haven't been able to determine.
You say that your rebuilding/restoring a 1942 C30 LAAT. Here is something that may help to identify it's markings:
49th L.A.A. Regiment, R.A. (TA), formed July 1940 - Home Forces, UK - July 1940 to July 1942, than transferred to 78th Infantry Division.
121st L.A.A. Regiment, R.A. (Leicestershire Regt)(TA), formed February 1942 from 44th (Leicestershire Regt) Searchlight Regiment, R.A. - Home Forces, UK - February 1942 to March 1944, than transferred to 2nd Army.
Still looking Keith.
Cheers
