I don't want to muddy the waters, but last year I restored a one family owned from new 1953 Meteor 700 twin. It had a 7t followed by three digit number. Among other things, the big ends were worn and there were shell bearings in the conrods of this machine. The crank was reground and new big end shells were fitted.
After working on an early '500 Twin', I was of the understanding that the integral big end type conrods could be replaced by the separate big end shell type ones. I also remember having a hell of a struggle splitting and reassembling that 500 Twin's crankcases, due to lack of clearance for the small ends of the conrods. If I remember rightly, I put the crank into the timing side main ok, but I think I put the drive side main on the crank mainshaft, dipped the small end of the conrod under the cutaway in the drive side case and then fed the whole main bearing into the drive side case, which was pre heated for this purpose.
I have added some pictures of the conrods from both these machines, if they might help in any way.
B.W.