Hello, Ady,
"System 11 coin door" isn't necessarily accurate - there were three different types used over the years. Williams were starting to use Sentinel/C220 ('ECA') coin doors with the later System 11 games; the first u.k. spec. game I remember with them was Riverboat Gambler, but I think other territories (particularly France) had them earlier*. The older type cash box and its attachment returned, as the 'all plastic' one found in mid-era Sys 11 games wasn't suitable. The Bad Cats which I unpacked had a specially-punched metal lid for the cashbox - the centre slot was much larger than normal, even though it had the 3-slot S.10 door which was standard for the u.k.
Re. the alterations to your game; is the extra circuit board really as used in Roadshow?. I can see that causing difficulties with coin inputs. The Wpc system has four separate coin inputs, while System 11 has three. Referring back to R/boat Gambler, the extra circuit board was on the door, and supplied power to the acceptor with control of three coin channels (the factory-fitted acceptor wasn't even programmed for the 20 pence coin, which wasn't on the coin entry label).
* conversely, some territories stayed with 2 or 3-slot doors into the Wpc era. Apart from the Yankees themselves, Germany and Holland held out for a while.
. Big family favourite so I wanted to ask if anyone has a coin door they would sell as it currently has a smashed up door which looks more like the plain Bally door and later coinmec? 
