It looks to me like it's a (relatively) quick change shell holder base. It eliminates the flathead screw that anchors the as-issued base, so just removing the two guide screws would let you swap a second base in. The sliding plate springs are retained in tunnels so they don't get loose or hop up and push the jaw plates out of the base. With four of these and two sets of S and LS plates, you could have all four possible shell holder head diameters available to pop in without having to fuss with anything other than the two guide screws.
Keybear,
I've been using one of these presses for 20 years and think they are great. The extra post-sizing steps I use for rifle loading (decapping before initial cleaning, post-sizing plain corncob cleaning to remove sizing lube, annealing and trimming when needed) has always made progressive loading less attractive than batch loading for precision rifle. The Co-ax die change system is perfect for this.
Note that the priming tool on top of the press is the only one made that I am aware of that forces primers to a fixed seating depth below flush with the case head. This can reduce velocity variation in rifle loads, sometimes significantly. I usually pre-seat primers lightly with a tray-fed hand tool while watching TV, then run them through the Forster press for final seating. This is especially good to do if the ammunition is for a floating firing pin gas gun to help minimize the chance of a slamfire. The hand tool pre-priming eliminates handling the primers one-at-a-time and as a result winds up being a tad faster for me in the end, despite handling each case twice. I also find the quick change die system makes extra steps less of a chore. Expanding rifle case mouths with a Lyman M die to improve seating straightness, for example.
You'll find liability warnings about loading for friends are pretty common. I always think it's best to let them learn to load for themselves on my equipment under my supervision. They then typically want to be able to do it for themselves whenever it's convenient and are inspired to get their own gear and join in the hobby. It's a good way to spread the interest in shooting sports in general.
