few people have asked… these are the ‘peltor/milspec’ plugs.

the plugs are Nexus TJ120 series.
http://www.pambry.co.uk/fourconductor.htm

The Nexus plugs are American NATO and not the UK type, they are smaller than the UK NATO. UK NATO are still readily available from people like Adams Aviation, Light Aero Spares etc so you can make up your own lead to what ever length you need.
Have you tried MART Aviation at http://www.martaviation.com, thet have ready made extension leads in stock, or at least they still have them listed!!!
I have a couple but they are still in regular use im affraid:)