[Arran]

Well a 50A5 is basically a loctal version of a 50L6 so the pin out is a little different but they are electrically the same. The 35Y4 is basically the octal version of a 35Z5. It shouldn't be too difficult to sort out by using a tube manual. It's very likely that the two tube changes were the only difference they made to the circuit between the two codes, otherwise there would be more documentation, did Riders show the changes made with a sub diagram? What I find strange is why there is a code 121 and a code 125 but nothing inbetween, most have a code 121 and then a code 122 but they didn't always do that for some reason.
Regards
Arran