Hi Merkle
Just a small point but the 6A number is not a serial number…it is the section/reference number for the RAF stores system.
6A is the section( 6A = instrument or elec )

1548 is the reference number which identifies the actual item.
If one googles 6A/1548 it does come up with a pic of the instrument even though its description may be ‘iffy’

cheers baz