Your cylinder head is an aftermarket service part that was cast by a forging shop in June of 1940. I have had several heads with casting dates on them, and they have all had the stock compression ratios, 4.2:1 for the Model "A" and 4.6:1 for the Model "B". There is no apparent feature on the current 5.5:1 and 6:1 reproduction heads that shows them to be other then stock.
If the water pump attachment has 4 holes, the head is for a Model "A". If the head has 3 water pump attachment holes, it is for a Model "B".