This is quite a difficult one to call as the pictures do show wood
that is not heavily knotted, is quite straight and tight grained

The end grain looks decidedly woolly to me, unlike e.g. European
Redwood, where growth rings are sharply defined. I still think it's some
kind of spruce, not unlike today's studding but from a larger section.
If so, it's a difficult timber to machine cleanly and won't wear well.

, but
is also from trees of quite small diameter such that it will have a
tendency to cup. Add that it is not T&G and I think you might have
problems with it as the t &g do tend to hold the wood flat - you nail
through the tongue.

This stuff is 12" wide, so I assume it will be for 2 x 6" boards.

If you do go for it I would recommend finding first a joiner with a
spindle moulder who is prepared to cut the tongues and grooves. That
may not be too easy on second hand wood as it may contain nails.

He'd be sharpening the cutters every five minutes if it's spruce. I
think that's one of the reasons it's essentially a structural timber.