[rossmum]

The first batches were definitely armourless, but I think you'll find that armour was brought in as standard around the same time things like the laminated glass windscreen began to take hold. I'd imagine that it still wasn't as armoured as the later 109E models (the Mk.II, as vanir has pointed out, carried equivalent protection to an E-4 having had an engine upgrade to balance the added weight), but by no means was the entire Mk.I production run armourless as a rule rather than the exception. Armour would've been fitted to older aircraft as they were sent in for repairs or refitting, the same way rifles would be retrofitted with improved parts if they were sent in for whatever reason. Some aircraft may well have gone through to 1941 without needing any major work, thus avoiding the retrofitting of armour until then.

The problem is that the Hurri and Spit I both varied so much from their entry into service to their replacement that we would need several new aircraft slots to properly model them. 2-blade fixed-pitch airscrew, 3-blade variable-pitch, 3-blade constant-speed, no armour, light armour, improvised armour, so on and so forth...