Heater muffs (or whatever)
Warmtoast, Danny et al - Surely this was a commonplace method of heating piston-engined aircraft? The Dakota had a similar set-up, still does in fact as can be seen by a glance at any specimen still flying now, and during 700 + hours on type never (to the best of my knowledge, anyway!) leaked noxious vapours into my aircraft.
Presumably the maintenance schedules called for regular inspection, but quality of material used would have been the most important factor. I suppose stainless steel would have been best, though doubt this was used as the visible parts were always of rusty appearance.