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STC Deltaphone (Trimphone Prototype)

The oldest example of a Trimphone in my collection. I've not really been able to find any info about this. I've found two photographs in the BT archives, one of which is a telephone identical to this and the other is the box for it. As you can see the circuitry inside is completely different from that of the final model (2/722) and is based on a 706. This was not a GPO release and does not appear to be a 712 either but is very similar to one. The earpiece for the handset does not have have a screw to hold it in place unlike the 722 or the 712 which is the most obvious difference.