Hi Nigel
What you appear to have there is a very early home built unit with what i think is the early version of the ARF shield, glued inside the casing, just disappearing out of shot at the top of your photo.
If I am correct, you basically have two options: (1) Remove the old ARF unit & purchase a new RF Bridge with licence, from the PilotAware Hardware shop and fit this. The big disadvantage with this option is that your older Raspberry Pi will not support the Audio warnings or the Mode C/S bearingless Targets. (2) The option i would recommend, is you confine that unit to a box of bits that you might find another use for in the future and purchase a complete Classic Kit From the PilotAwae shop and you will have a fully functioning, guaranteed unit, with a years licence for £192.00.
Probably not quite what you wanted to hear but I hope this helps.
Alan