Britax Twin-Lok
Attachment:
Britax Twin-Lok seat belt.jpg
Attachment:
Triumph Stag Seat Belt Receptacle.jpeg
Here is an add for the Twin-Lok used on Triumph.
Attachment:
Britax safety belts 1970 original advert.jpg
You would need to be extremely lucky to find one NOS. There would be some out there but i doubt very much if the owners would ever let them go. If you are looking on Ebay for a centre receptacle then try expanding your search to Triumph as well. They look very similar.
Sorry i can't help with any part numbers. Hopfully someone on here can check their clubby parts book.
If you had an old set that needed new webbing there are still 2 businesses doing the old style re-webbing.
It would be alot easier and cheaper to go after market however I hope you can find one because your GT is very original.