Hi Richard..
The number on the left below the barrel is the engine number...
'WM20' as an engine prefix denotes a wartime WDM20 engine...'W' was used from 1940-45 as a prefix letter....
The numbers on the front of the crankcase are the crankcase part number and (7306) the numbers showing the two crankcase halves are a matched pair...
The 'M65' is a quality inspectors number stamp...
Regarding the chassis 'Z' was the year prefix for late 1949-1952 (inc)....This prefix covers rigid and plunger frames...The actual year and frame type can be determined from the number itself...
Saying that though...If, as you say, the frame number starts ZM21 (as opposed to ZM20) then it has had the number altered at some point...
Frame numbers for both the M20 and M21 commence with a prefix letter and the number for the smaller model..eg 'ZM20'..
Use of the frame number prefix from the smaller model for both capacities was normal BSA practice during the late 40's and the 50's (the one notable exception was the Rocket Gold Star that had a 'GA10' frame number)....
Even if the bike had its original (1950) engine, if that is indeed the year of manufacture, it would still have a cast iron head...The alloy head was introduced for 1951....Ian
email (option): ian@wright52.plus.com
Hi Richard...read the edited version of my post...I have made some alterations..
For a given year both models used the same gearbox with the same internal ratios...Gearing changes for the two models were achieved by changing the engine sprocket size...For this reason there are no numbers on the box other than casting numbers...
The markings on the engine do not indicate anything about its history and whether it has been rebuilt. The quality control inspectors stamp was applied in the factory at the time of the manufacture of the engine...
The only way to tell conclusively whether it is a 500 or 600 is to remove the cylinder head and measure the stroke of the crank....An M20 has a stroke of 94mm, an M21 has a stroke of 112mm...
The two engines share many components (most actually), the only difference internally being the stroke of the crank and the piston specification which was altered to suit...
Both models also used the same frame, so either engine can be fitted...It's the engine that determines the model...All postwar M Series frames have the sidecar lugs (I'm assuming yours has telescopic forks?)...
These models were more heavily constructed than the OHV B Series models and were intended to be strong enough for sidecar duty...
Due to the simplicity of swapping engines of different capacities and from different years having them 'mixed up' is a very common occurrence...Back in the days when you could pick up ex WD engines for a few pounds (and also civvy ones if they came along) it was often cheaper to fit another engine than overhaul a worn out one...I did it myself many times during my teens and 20's before people started to worry about things being 'correct'....Ian
email (option): ian@wright52.plus.com
I measured the stroke through the TDC timing screw on the head,
Keeping the barrel head on..
I bought an M21 engine for parts (Mainly for the long stroke flywheel)
And inserted a hard wire into the TDC inspection hole,
Marked the wire on top and bottom dead centers, and measured.
It was a 500 ! 
Noam.
email (option): noam10@gmail.com