is the diameter of the transfer port milled into the receiver. IIRC correctly, the 16J rifles have 2.5mm TPs, while the 7.5J rifle measures only 1.5mm.
The reason for the smaller TP is so the 7.5J rifle can complete 60-shot matches (plus multiple sighters) on a single cylinder, in its un-regulated velocity sweet spot, using the 7.4gr (avg) wadcutter pellets
used in paper-punching, 10m Int'l matches.
As others have mentioned, you can de-tune the 16J model to approach the 7.5J model's velocity, using the heavier, round nose pellets easily enough. But for actual 10m Int'l matches, you will be required to use the wadcutter pellets. However, using such lightweight pellets will only increase the velocity you've worked to reduce, and
increase air consumption due to the less mass of the lighter pellet. Probably doable, but you will have to work at it.
The standard strangle screw on the rifle has a conical shape. You might try a replacement set screw, with a rounded shape to match the back wall profile of the 2.5mm Transfer port. Essentially, you are trying to strangle the TP to almost no air passage, then back off as needed to build the minimal pressure needed.