I realise this won't help the original poster but I find this quite concerning since I recommend Record Power's chucks as a good option.
If Record say they have never made an SC3, their dedicated thread type chuck, then what is the source of the seller of the OP's chuck? I think Record will know what they have and haven't manufactured, in which case this item is fraudulent and the original poster would be entitled to a refund, based on UK law and also Ebay/Paypal's rules for sellers.
I also wonder if anyone else has bought one of these chucks. i.e. owners of lathes with 1"x10 headstock threads. Union Jubilee springs to mind.
It is a shame that the original poster didn't purchase a chuck with the correct thread to start with. That would have removed the need for an adaptor, with the inherent problems of shortened bed length and possible misalignment.
I will be watching this thread with interest.