In the early 90s I was working at Filton, and did a bit of work on the MTOW increases. Long time ago, but as I remember the sequence:
Basic prototype was 66 t MTOW
-100 series went to 68 t.
-200 went to 73.5t.
At that stage some customers started asking for an increase in MTOW. The request was to do with trans-continental flights in the US and Canada, where extra fuel was desired. The analysis of the 73.5 t wing had suggested that it could take more load, so an analysis was done at higher MTOWs. An initial analysis was done at 76 t, but the results were marginally below allowables. The anaylsis was then run at 75.5 t, which gave acceptable results. As I recall the 75.5 t allowance had some conditions attached, such as a different MMEL and some cg restrictions. Not all airlines took the 75.5 t option, due in part to the increase on field length required.
At that time the A321 was in planning, and there was talk of an increased MTOW version of the A320 using the A321 wing. I think the idea was to have a common wing for the A320 /A321 series. The A319 was still only a concept, the marketing people said there would be no demand for it.
The analysis that was done to confirm the 75.5 t MTOW was done using basic finite element methods, plus "hand-stressing" methods. It is possible that after I left the analysis was re-run using more accurate methods, which may have allowed an increase in MTOW.