Actually, S&W hand ejector (swing cylinder) revolvers did have hammer blocks from about 1915. There were two different types, but both moved out from the sideplate rather than up and down. Both suffered from the fact that they were not "positive", but depended on spring tension to operate. If the spring broke or failed, or gunked up from dirt, the block could fail to move out to block the hammer. Converting a gun with either type hammer block to the modern type would be even more difficult than converting an early HE that had no sideplate cuts at all.
Jim