12g940 means it's a 1275 head. There are differences between A+ 12g940 and A series 12g940. Here's an extract from the retro minis UK site
12G940 1.401/35.6 1.15/29.2 21.4 Stamped 12G1805 behind thermostat - MG1300, Austin 1300GT & Mk3 Cooper S
12G940 1.401/35.6 1.15/29.2 21.4 1275GT, Austin 1300, MG Metro, Late 1275 A+ & Turbo (note 2 & 3)
12G940 1.312/33.33 1.15/29.2 21.4 Early 1275 A+
The first set of numbers are inlet valve size and the second set are exhaust valve sizes.