Having blown the box on Sunday, I bid and won on a new LT77 on e bay (hmmm, 300 miles, Slough to Doncaster, will it be a load of $#1t?)
The existing jammed in limbo between reverse and first is serial number:
52A0-----D
The new one is:
50A0------D
50A is correct for pre Tdi 90, but...