Hi Den,
Yes, they were all UK releases. As you rightly assumed, they weren't for general release but were produced for the Juke Box industry. 36 releases are known in total. The first release was "Moonlight Gambler" by Frankie Laine (JK 1000). All of the 'JK' series were, of course, only publicly available as 78s with a PB number prefix - with one excption being the final release; "Kisses Sweeter Than Wine / Rock-A-Chicka" by Frankie Vaughan (JK 1035) which, for some reason, was the only one also released as a stock 45 (PB 775), as well as a 78 with the same number.
The series was dominated by MOR artists such as Guy Mitchell, Frankie Vaughan, Rosemary Clooney, Johnnie Ray etc, with the only rockin' release being "Start Movin'" by Sal Mineo (JK 1024). Because of their intended usage on juke boxes, very few have turned up in decent condition. The majority that do turn up for sale are usually VG+ or less.
Here's a label example for you to give you an idea of how they looked (one I sold on Ebay some months ago).
Good luck with the Treniers and Doc Bagby 45s on Fontana by the way.