Kdean, The mark stating "Union Cut. Co., Olean, NY, USA" you are asking about was first used in 1912. After 1923 this mark was still used, but another blade would likely have Ka-Bar stamped somewhere as well as Union Cut. Co., Olean, NY, USA. My guess is your knife is circa 1920, so it is old.
Please keep in mind that I am a knife collector, NOT a knife appraiser. The picture of the knife you are asking about is dark and difficult to evaluate. Here are my observations on the points you mentioned:
1. Early / Old. Yes
2. Rarity. Not especially. It is old which makes it rare.
3. Valuable. This is a difficult question for any knife collector to answer, especially without any parameters on what defines "value". Some folks have a knife that is priceless to them because it belonged to grandpa, but the same knife is worthless to someone else. To some people $100 is valuable while other folks may have higher and lower thresholds. Whatever value your knife has is diminished by its condition. The blades look reasonably full, but are corroded due to neglect. Union as a brand has a following so there will likely be some collector interest. I would expect the interest to be actionable for a 2 digit price, less actionable for a 3 digit price, and non-existent for 4 digits or more.
I enjoy trying to help fellow knife collectors. If you are trying to work out how to sell the knife, my best advice would be to put it on eBay with a starting price of $0.99 for a six day auction ending on Sunday evening between 8 PM and 10 PM EDT and let the market decide. Be sure to use the key words of "Union" and all of the words in the tang stamp so it will match to keywords that Union collectors will have in their searches.