1. The Model B head made for a Model B engine has (2) "extra" holes to match the (2) "extra" holes in a Model B block; for which these (2) "extra holes are not found in Model A blocks or heads.
2. Don't remember seeing extra holes in a Model A Police Head which is marked "B" -- maybe missed them; however, years ago, I mixed Model A & Model B heads, gaskets, & blocks in "all" combinations & never was told that the (2) "extra" holes had to be plugged & must have been extremely fortunate because I never developed leaks.
3. Before removing a head & installing any "borrowed" head, or "used" head, always a good idea to check for "flatness" -- especially with a Model A Police Head marked "B" with increased pressure in the combustion chamber.
4. Vintage mechanic's lesson heard years ago for a typical method for a rookie to warp any Model A or Model B head:
A. Run engine until attaining operating temperature, & stop engine.
B. Check oil on dip stick.
C. Observe milky colored oil, i.e., water mixed with oil; or, observe no water in oil.
D. Get a wrench & immediately remove head before it cools off; plus loosen head nuts all the way one at a time, while head is hot, as opposed to first waiting for the head to "totally" cool, then carefully loosen nuts in reverse order of recommended sequence for tightening.
Hope this helps.