[skidlids]

The confusing point is not all 748R's run the 15mm offset discs, only the ones with the Ohlins forks, my 2000 model 748RS came with Showa forks and 10mm offset discs same as the 748R of that year

Your pic does indeed show a disc with the 15mm offset for use with the Ohlins forks, when bolted to the wheel they will measure 130mm from outer edge of one disc to the outer edge of the other disc, where as for the standard setup that goes with most 748/916/996 and the Showa forks that same measurement will be 120mm
Note the standard Hub width is 100mm