Hi, welcome to AAPK
The information you found most likely refers to the tree stamped on the blades of most Boker knives. It may not be applicable to the tree stamp you see on the bolster of this knife.
I have a few of these. The long bolster identifies the knife as a "barlow" style jack knife. Boker knives are difficult to date, and the barlows are even more difficult than most. It's difficult to tell from the photos, but yours does appear to be an early knife.
A better way to tell if this knife is old is to check the handle material. If it is really old, the handle wil be bone. If the knife was made after 1965 or so, the handle will be made of delrin, a plastic material. If you look really close at the handle material with a magnifying glass, you will see little imperfections and natural pits in the bone handles. The delrin will appear smooth.
Another thing you might try is the "hot pin test". Heat up the tip of a needle or pin with a match until it turns red. Try to poke the pin into the handle. Do this in an inconspicuous spot. If the handle is bone, the pin will not penetrate. If delrin, the hot pin will make a small hole in the handle.
Now, as to value. I am sure you will value it because it belonged to your Grandfather. This knife sells on ebay - whether bone or delrin handle - in the $15 to $25 range. These knives aren't selling for a lot now, nor did they when first made. The Barlow style was created to be a low cost, reliable knife.
Take a close look at the blade. There is probably a tang stamp that reads "493". That is the knife pattern number. BTW, this knife was made in the USA.
I hope this helps.