Those appear to be early or mid-century English Windsor type chairs.......... the parts were often mass-produced by chair-bodgers in the woods as the trees were cut and and made up and often sold by the dozen or gross.
It's hard to say what finish is most appropriate as they were often sold, as I mentioned, by the dozen or gross and often sold as is - unfinished so that the purchaser took them home and painted them to suit.
Linseed oil's been mentioned..... in those days it was up to the user to decide. most were painted or sometimes left unfinished, which could well be the case if they were sold to the War Office.
I would expect the seat to be Elm and the legs either Ash or, most likely, Beech.
There is one interesting point that may (or may not) be relevant in the third picture showing the leg/stretcher. The disassembled stretcher seems to have a groove just inside the joint. It may be a one-off on that joint, but it would be interesting to see if the other stretchers had a similar groove just inside the joint.
This may be a one off...... or the original bodger would have turned the legs with the wood wet- straight from the tree - then turned the groove in the stretcher, hammered it all together and as the legs dried, the shrinkage tightened the joint. You may also notice if the section of the leg is slightly oval.
Are there any old glue remnants present in the joints or were they assembled dry?
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