This is the bolt that goes through ARP1073 spacer on lower radiator mount on classic Minis from 1974 with a one-piece cowl. 5/16" bolt, 2.2502" long, with small amount of 1" thread at end. Ref: 12G4270.
See 12A2153 for OE-style lower radiator bracket mount only, or 12A2153E for aftermarket bracket that includes the cowl bolt.