The plot thickens!
The distance between the seat runners in my aeroplane - a 1972 FRA150L, serial 0136 - is 12";
That on a 1974 FRA150L serial number 0244 is 11";
That on the seats I bought (putatively a 1974 F150L) is 11".
Given the difficulty in finding a replacement seat, with my engineer's agreement and advice, I have had the broken "recline" mechanism on my aeroplane's Starboard set welded by a CAA (UK equivalent of FAA) approved welder, and a rather neat job he's done. The recline mechanism never worked so there's no loss of function.
Anyway, original "broken" seat has been repaired and is stronger than it's ever been.
A