Well, the key to that post, is the custom/field mod bits. When you consider out of the 15,875 ponies bult, that it wasnt factory fitting (to the D models, as were the reference in that post), the rare bit would be well justified. Personally, no matter what I was in, I would mount a mirror.
Hehehe ironically, even checking out the '51 at wiki - check out the lead plane:
As you say, its very simple. Hell, all you need is a drill, a wing mirror off an old Packard (ie) and a nut, and a bolt
And as I said in my earlier posts - a mirror for the 109E's - absolutly... the other models... unsure, I've seen more photo's of the E's with them than others.
But thank you for dredging up a past question I dont believe has actually ever been answered;
With external mirrors - Can they be modeled onto the 3D model of the aircraft their being added too...?