Not quite understanding what you're trying to say here. First you say you disagree, then say the fir floors are original.
I would respectfully disagree with Gary about this being an original subfloor. They were probably originally exposed original floor
Are you saying that these fir floorboards are installed over an existing subfloor? That is quite possible. Here on the pacific coast, it was commonly done both ways. T&G 3/4" x 3.5" fir floor boards were sometimes nailed directly to the floor joists. And they served as both subfloor and finished floor. In other areas, they were used as a subfloor only. And in other situations, fir flooring was nailed to solid lumber subfloors. But that was more uncommon here, as builders are a thrifty lot. Mostly, fir flooring was laid as both finished wood floor and subfloor. Seen it a lot in San Francisco and Berkeley. Then in the nicer areas of the home, they would overlay the fir with 5/16" face-nailed oak, usually quartersawn white oak. This was standard practice in the SF Bay area from the late 19th century through the 1930's. Seen it in multiple homes.
But in stores, warehouses, schools, factories, etc., it was used as both the subfloor AND the finished floor. It was often just oiled with products like P&L's Okene Oil. Or painted. In homes, they would try to stain, shellac and wax ( the typical finishing system for hardwood ) but it didn't work as well on fir as it did on oak due to fir's greater denting.
BTW, the predominant fir used was Douglas Fir. It is known to be strong and somewhat rot resistant, if protected from water by paint. Not nearly as rot resistant as redwood or cedar but much stronger. It has been the building industry's framing wood of choice for over 70 yrs.
Clear fir boards have been used as flooring, interior trim, stair components, furniture, exterior trim, cabinets, etc. A very nice wood.