Yeah Glosters were in the Hawker Siddeley group so a similar part number system might be used.

The 26 prefix is for reference numbers and used to be called section/reference numbers – 26 being the section – followed by the reference number.
In the AP system there was an ‘index of part numbers book’ which allowed you to cross reference a part number to a reference number.
Conversely there was also an ‘index of reference numbers book’ which you could use to cross refer a reference number to a part number.
Later on when the nato codes started to be used (NSN – Nato Stock Number) the last 7 numbers of the sect/ref were also used as the number for the NSN

So 26FZ 1234567 might become 6115-99- 1234567 [NSN] (completely made up numbers but hopefully you get the idea)