421_print = has 2337 E on back in pencil

424_print = has 2337 I on back in pencil

425_print = has 2337 N on back in pencil

426_print = has 2337 C on back in pencil, and
'1967' in biro in what looks like Barry Hope's
handwriting.

509_print = The 509 is something of a mystery item. It seems to
have consisted of a modified 500 range tuner built into
a PA system with a valve power amp, etc. However I've never
seen one, and no-one I knew at Armstrong could recall anything
about it!

500_range_Olympia_Show_12744p2 also has '2' written in pencil on back

Note that the "delivery note" for the 500 Olympia Show
dates the event as being in 1972.