The MEC 600Jr in 10 gauge that I have is settable to either 3-1/2" or 2-7/8" by the same mechanism that the 12 and 20 600s use to go from 3" to 2-3/4".

The center post, where it mounts into the lower frame, has two threaded holes in it. Juggling between those two threaded holes in the post and the two holes in the base gives you the option of two shell lengths.

I was surprised to find that the 10 was set up to switch between 3-1/2" and 2-7/8", but most pleasantly so. Light ten loads are great!