Think you might be right about the military thing, in that perhaps the military ones all had flat panels ... but maybe flat panel heaters were also fitted to civvy models. Don't think the S2 I had had a military background, but then again I don't know if my heater was an original fitment to it either!
I'd LOVE to swap the heater for a decent late 2a grille (cross-shaped galv mesh, not the inverted T-shaped galv mesh) ... if anyone out there is reading this ... and I'm also after a usable/repairable front panel off a late 2a or 3. (SW Wales).