Cut-outs in the lower corners of the glass:
Also quite a gap between the lower corner of the seal and the lower corner of the side rail. Neither leaks though.
My 73 screen frame (left) goes down then makes a turn to meet the bottom rail, whereas the later upright goes straight down to the foot (yellow arrows). There seems to be a definite kink in the later upright to enable that to happen (red arrow), which means the glass would have to be narrower below that point. A strengthening measure?
Moss AHH8227 for 1962 to 1972 on the left, HZA5415 for 1972 to 1980 on the right. Both have 'OE' appended to the part number and claim to be made from the original Triplex tooling (at well over £300), the later glass having 'recessed corners' to 'minimise breakage' at 'A'. I suspect these are to prevent the breakages that have been mentioned in the past where screws that are too long have been used to assemble the frame. Both appear to have the same kink at 'B' which is needed for the later upright. They also have 'non OE' versions of both screens (at £65), same dates, with no recesses on HZA5415, but apparently the same overall shape: