I recently purchased a 1918-dated Model 1917. The grips appear to be original but look like they have a satin finish of some type applied. Does anyone know what the original finish process was back in the day? I assumed it was the standard BLO or whatever they used back then but without any sort of varnish or shellac top coat. Help please!

I think your assumption is pretty good. Of the 1917's that I have, and have had in the past, I have detected no varnish type of topcoat. A BLO bath makes sense to me. I am sure others will be along with much mote insight than I can offer.
Give us some pictures! There are a lot of us that really appreciate the 1917 here.

These stocks have seen very little use and show no evidence of a topcoat or sealer.
IMG_3033.jpg

Will do. It is still sitting at the FFL waiting for me to get the thumbs up from ATF or whoever it is that approves transfers. Apparently there is a significant backlog.