Have they moved the mast base "slot" position further back in modern boards compared to those in the 90's? I suppose I could have a look couldnt I but the board is packed away & Im lazy..I just noticed my 2003 bump & jump board has a tendency to round up when I waterstart & after gybing more so than my 96 waveboard.Ive adjusted my sailing to suit but I was thinking about it last night & seem to recall reading something about the mast track position being further back these days..
If so whats the logic behind it?

More stable draft, C of E nowhere near as far back in sails now.

Also related to the widepoint of the boards coming back and vee changes as well.

Sboard - the rounding up may be more due to lack of volume in the tail?

The trend was to make sails with wider wind ranges and increased ability to hold gusts. The loose leech brought the COE down and forward when raked back. By bringing the mast track back they could compensate for the difference. However, once the boards became wider and the volume moved back under foot, the mast had to move back closer to the center of the board's volume. This started in the early 90's. Boards made in the late 80's had the mast track furthest forward. With a wide tail, however, you need a larger fin and deeper water. Nevertheless, you can hold a wider range of sails this way. Finally, since most windsurfers weren't interested in jumping, the looser de-powered sails sacrificed a jumping ability especially with the mast track moving back. The wider tails also discouraged jumping in that it's safest to land a jump stern first. The old narrow stern boards were more comfortable in a landing.

could it be different fins in either board or fin position if your old board is a us box?

A US box is a type of fin box. It was the most common fin fastning system until the bolt through systems such as the Tuttle and Powerbox systems were developed in the 90s.

The US box system uses a tab on the fin and a small bolt that screws into a plate that can slide up and down the box in a recess. US boxes are still commonly used for mast tracks.

Yes the 96 board has a US finbox.