A standard M12 metric nut will not fit
If it is a 12mm thread it will be a M12 X 1.25 thread pitch.
Probably your best option would be to vist your local steel stockist or engineering fixing stockist ( Take the joint with you ).
Either should be able to give you an exact answer.
Both 7/16" UNF & M12 X 1.25 are available on ebay
