Ha ha, if you wre as old as me you would remember that type of instrument here and there in the seventies - they certainly weren't rare. They seemed to be concentrated in LNW boxes rather than other boxes on the LMR with LNW instruments, and mostly in larger boxes from memory. Did not that type have different legs too?
The two-tapper type were typically found where different boxes worked Up and Down lines - both at large stations and at places like Talacre - https://signalbox.org/diagrams.php?id=1146
I suspect (without looking) the arrangements for converting Permissive to Absdolute Block is found in the General Appendix under "Working of Passenger Trains over Goods Lines" and it entails sending a Block Conversion Ticket through on the last train before it is required. The ticket tucked nicely behind the flap on LNWR instruments but not, unfortunately, on the early BR standard instruments that had a similar fitting..