Just for the record, the chassis stamp on slightly later Arch chassis is also on the rear suspension mount but in a different format. This is my car (not very clear as has been repainted) but it says AM90 at the top (Arch Motors 1990) and J2 on the side for Jig 2 (they have/had 2 different chassis jigs, no doubt with slightly different tolerances, so this is to make sure the chassis goes back on the right one if it's ever back for repairs).