I cannot find any reference to square or round safety pins in my manuals or drawings. Normally that level of detail is not placed in manuals so it would have been lucky to find it. I have attached a portion of the engineering drawing for the M404A2 fuze, but it does not address the pin shape. Neither does the drawing from TM 9-1950, Feb 1958 which I also attached. The M28 (T80E2) HEAT warhead used the M404 or M404A1 fuze. The M28A2 HEAT used either the M404A1 or the M404A2 fuze. The spike nosed version, the M35 (T205E1) used the M408 and the M35A1 HEAT used the M408E1 fuze. The M29A1 and M29A2 Practice rounds used the M405 fuze. The Practice M36 (T206E1) version of the M35 had no fuze. The T127E2 Smoke used the M404A1 and the T127E3 Smoke used either the M404A1 or the M404A2 fuze. The manual mentions nothing about the differences in any of the fuzes. The difference between the M404, the A1 and the A2 could be slight design changes (such as a square versus round pin), different materials used, etc. The drawing from the manual shows a round pin for what that is worth. The M405 is basically an inert body assembly of the M404 series fuze. The manual states that it is an inert fuze similar in appearance to the M404A2 to simulate handling and pin ejection. It does say that it uses a bore-riding, round ejection pin. Sorry but I can't pin it down. To the best of my recollection I do not recall seeing a square pin on a US round, but when I was dealing with them that was a level of detail that normally I didn't get into. The only example I have is an M-1 HEAT version made by CEV in Brazil. It is basically a copy of the US M28 and the pin is round.