Yes you are correct the Sizemaster can be adjusted as you have suggested for 2 7/8" shells. I assume it works the same as the 600Jr.

The main upright bar can be adjusted for height by repositioning the large hex-headed screw located in the base beneath the loading surface.

The 10 ga 600 Jr has the holes in the bar and base set up by the factory to enable a change from 3-1/2" down to 2-7/8". All you need is the correct sized wrench.

I use a VersaMec in 10ga and had to use the "short kit"