FYI - I have BT-21 Chassis no. 23. Though the Arch Motors number stamp is long gone from the frame (assume from some amount of contact with the armco over the years by previous owners - though I shouldn't talk having not helped matters by spinning her into the pit wall at Brands last July
), the FIA papers and history indicate that the car was built in 1967 as a screamer and since then converted to twin cam spec.