Its been said that Enfield made up a batch of Envoy-marked receivers (possibly 800 or 1,000) and then just built them up as and when. Hence the numbering not correlating to the assembly/sale date.
They had to mill off the "Envoy" markings when they came up with the Enforcer and built the first batches using unsold Envoy receivers - ie the ones with the milled ovoid area.
Does anyone have the Enfield data? Presumably there were about 6-800 Envoys built and sold, 200-odd Envoy receivers built as Enforcers, and then about another 500 fresh receivers prepared for Enforcers.