Hi Dave...They are cast iron flywheels (as opposed to the forged steel WD M20 wheels). They sound like M21 wheels from the description. A measurement of 56mm from the center of the mainshaft to the center of the big end will confirm this.
The 600 and 700 series numbers are confusing to say the least and an extensive look at the subject did not produce a difinitive result. 600 numbers are steel, 700 numbers cast iron.. Both materials were used for the M20 and M21. Material spec. was altered for different users, so often flywheel sets of both materials would be in production concurrently for the same model. All postwar OHV B Series engines used steel wheels only and in the latter stages of the various models production the M20/21 and B33 were standardised around the same steel forgings (66-635), which had the stroke machined differently for particular model applications. The exception was the B31 which had its own, lighter, forgings.....Ian
Spot on Ian they measure 56mm between centres for big end & main shaft.I know what you mean about BSA part numbers all over the place.Who else would produce a part which has a cast number that is one digit away from the actual part number (-pre 1960)
.Handy to know about 600 & 700 numbers for flywheels one being cast other steel.Now thinking about another project using these flywheels but better put on hold as I have 2 on the go at moment, I bought the complete motor with the oval flywheels a BB34 just need to find a suitable frame. BSA addiction is for life 
Dave.
That sounds familiar Dave...I can think them up faster than I can do them as well... ..Ian