Given the year (the change was in 2007), your chassis is almost certainly Arch made and imperial. The easy way to recognise metric vs imperial is to look at the lower horizontal chassis tube in the rear wheel arch, the one that the rear ARB is bolted to. If it's round for the full length, it's imperial, if it's square halfway, then changes to round, it's metric. Assuming your chassis is imperial, it will have AMXX stamped on the seatbelt mounting boss, where AM is Arch Motors and XX is the jig number used (bet it is 02).