Other than lacking the "hole" for the ErgoBrain button on the 1998 shifters, I haven't paid enough/any attention to the difference with the post-1998 brake lever handle bodies ...
One of these days, I'll check more closely ...
AND/OR ... maybe, one of these days I'll see if a 10-speed indexing wheel can be retrofitted into the RIGHT 1998 shifter body.
BTW. I think that I had observed some minor differences in the "clockwork" spring part of the assembly in various post-1998 levers, but I don't know if that is a variation in the Record-to-Mirage components OR (without looking) a bad memory & confusing the variation with the older 8-speed style's assembly.
AND, that's another example of why I think Shimano's Customer Service is so great ...
BUT, of course, it is disappointing that the STI shifters become expensive paperweights if/when they stop working.
Maybe the next generation of Shimano shifters will (finally!) be rebuildable.