Standard is small 1/4"x3/8" hardware bolt as used on classic Mini wheel cylinders, timing covers, and 1974-on one-piece radiator housing. SAE thread. Sold each. Ref: HDWB2742, SH604031, 7H7250, SH604031M, ARA1117M.
Note some replacement radiators that we have seen use metric threaded screws for securing to the housing, select HDWB2742M ("MET").