[tankbarrell]

All Valentine gun tanks have gearbox/ radiator cover that are made up of channel sections and are essentially flat in section. Archer has a different style which are made from plate and are triangular in section. I suspect the solid rear plate is to act as a blast shield.

These are clearly visible here and I still say it's an Archer with a dummy or cut short gun.

The first pic is from the Archer service instruction book and the second is from the Valentine X and XI book.