What you see is a Choate folding stock for a moss burg 500 attached to the back of the PC9..... Choates will work, Butler Creeks won't. I and a couple other folks around the country have done this.... and a few folks have even done an adjustible stock like on an AR...
I'll try to explain.... take the 'workings' out of the stock... just the standard field strip.... what you will discover is there is a long bolt up through the stock from the butt that connects to a metal frame work in the back of the receiver.... remove this bolt and then you can cut the back of the stock off (about 1" back from the metal) and install a Choate folder..... the difficulty is you need a shorter bolt for the choate.... i think it is a 1/4" fine thread and I had to buy one slightly too long and then cut and refinish the end threads... also you have to heat up the connecting end of the choate, so you have to remove the plastic stock and just use the metal frame... a then press the metal into the PC9 cut polymer part and put the short bolt on and pull it all together. Keep in mind when you cut the PC9 stock off that is it... one more leap of faith.