Well, where should I start? I just got reminded that I could share some more information. About the "split plugs" used on the shoulder stock of the ESB blaster. "Split plug" is already a term that only confuses the heck out of some folks that still have those lying around somewhere. What's often referred to as a split plug is nothing more than a connector/plug that was "split" open so it could be attached flat to the shoulder stock. In order to locate these, a bit more information is always helpful, so here it goes.
The ones that I have located were manufactured by a german company called "Hirschmann". They are well known for their antennas, be it cars or household. If you look at the center of the plug in the picture below, you need to imagine that two little metal pins go through there to connect to your old radio or tv. Cause that's what they have been used for, to connect an antenna to your TV or radio. Having been able to find the originals I can provide you with the parts number (stamped onto the part), and I can also tell you guys that the manufacturer is out of all old stock, otherwise I would have offered them here to you guys. They have long been discontinued, mostly saw use in the 60's/70's.
The picture below shows the front and the back. The circle you see on the back depicting an "h" is the companies logo. There are LMK/AM connectors(pictured) and UKW/FM connectors(not pictured). They both look identical folded open, but the FM connector has a plastic pin on the back that needs to be cut off, but can be used as well, same look, everything, as you won't see the back on the stock anyways. These connectors have also been used on other Star Wars costumes. The part number for the FM connector was 730 854 and the part number for the AM connector was 730 851. The cable that was used on these plugs was a flat wire, not round as used on later connectors. In order to get an idea how the closed connector looks like, take a look at the backside, imagine that the area with the two holes is the bottom and to the left and the right is a view of the side. Mine came from the U.K. and that's the best place to look for them. Hope this helps you guys.