This is quite normal for this time frame. It is indeed referred to as the "lazy ampersand". The changeover occured within the 1973-74 time frame on all the frame sizes, so it depends on when the stamping dies wore out. The stamping die for the frame markings was probably changed over at a different time than the one for the barrels. The parts were made separately. It's important for us to remember that at the time, no one was thinking about these becoming collectable 40-50 years down the road. The parts were just grabbed from what was in the bin until the parts bin emptied out. There even may have been parts with both styles of stamping in the bin at the same time.