Door phone handsets and cradles

[Sparks]

Not long ago I bought a handset unit off eBay. Quite an old unit but probably unused judging by its condition. However it has no manufacturer's marks or labels or wiring instructions. It has a GPO No.3 handset fitted with a 'top hat' carbon microphone. Overall it's similar to a Sterdy unit but different wiring etc inside. I've attached a few pics of the mystery unit and some of a Sterdy for comparison. Does anyone have any ideas as to its provenance ? I did enquire with Sterdy and one or two door phone suppliers but none of them have claimed it. Thankyou.

The mystery unit is the first three pictures from the left.

[Sparks]

Any clues folks ? I dismantled and rebuilt it this afternoon; all I discovered was that the hook switch is made by Plessey. No names on the microphone or earphone elements but I'd reasonably assume these are A.P. Besson products. Overall it's a well-made bit of kit but a little shy of its origins. Perhaps a well-intentioned universal replacement unit or a copy of something plain obscure. One other thing is that apart from the hook switch and the pushbutton switch, there is no circuitry, PCBs chips etc. Just wiring.

[OscarFoxtrot]

The GPO style handset was used by Entryphone and they might have had models other than the 5901 shown here

http://www.intercomsrus.com/entryphone.htm

door entry intercoms were typically 4+n wiring with all handsets having a common, transmit, receive, and lock release wire in common, and n call wires, one per handset/button on the panel. You would normally have a hookswitch and buzzer in the base and that's it.

[Sparks]

Thanks Oscar. As I recall, Entryphone was one of the companies I contacted (along with Sterdy and Intercomsrus) but they denied knowledge of this particular unit.