Thats the one, reduces the barrel/breech size to .685........I want one cos I've got a nice 8" barrel that I like but it has a big bore and the paint just drops out the end of it.
Just order from G3PB, I've ordered loads of stuff from there before now.
They are excellent to do business with, give Terry a call, he'll sort you out!
me and q pid about five years ago had some of these made by Belsales, they were cocker threaded both ends and were a tight bore section about 1" long. not sure if these were one offs or not. sold mine with my old evo shame cos i've rejoined the cocker family now an it would be usefull. if i remember correctly they cost about 10 squids. contact belsales an see if they're still doin them or can do them for you