Rossin Performance Internal Seat Clamp

[hondicusp]

Hi, have just bought a Rossin Performance frame and forks that is missing the recessed integrated seat clamp bolt. Does anybody know what they look like/ how they work? I'm not expecting to find one but I may be able to engineer one if I can find out what I need to make.
Any help or photos gratefully received, thank you

 

[hondicusp]

This is what it looks like from the outside but I don't know any more