Looking at numbers, C5110 commenced at WM20 5001 and the first machines arrived at RAOC Chilwell on 22nd September 1939. It looks as if the first 2000 were in accordance with the contract and were numbered C3928419 - C3931195. This would make yours, probably C3929645...unless perhaps Rob Miller can extrapolate anything different from the 'Key cards'.
Almost correct Rik! But according to the Key Cards, WM20.6227 comes from a block within contract C/5110 where the census number is 1 off the "logical" sequence. (And there were more census number anomalies in this contract by the way!) So WM20.6227 would have had census number C3929646 on the tank. And from the factory ledgers we know that it was despatched on 2/10/39.