As far as I can read it is AM REF No 55/534, although the second "5" is not very clear. It may be 5 followed by a letter, as mentioned in
http://www.cpearce.orconhosting.net....M_LISTING.html
I don't have the pieces, only the photos.
Thanks.
Laurence
PS: I just saw a photo of bomb selector 5D-656 on
http://aircraft-cockpits.com/ww2uk/5...b-Selector.jpg
It has a number of small rings around the switches. Could it be that? If so, which aircraft?