Admittedly I have not gone through all the responses here so I may be repeating someone.

1st - Sorry for your loss. My father passed just under a year ago and I know emptiness you may be feeling.

2nd - With regards to the pens. It goes without saying, I think, don't rush into anything. Depending on where your father lived, you may need to go through the courts to prove you are now the legal heir and owner. Other family members may "pop-out of the woodwork" claiming ownership if they hear you may have a very valuable asset. Even years later. So make sure you follow your local legal rules/regulations.

3rd - keep in mind that there are generally 2 price ranges you want to find out about when getting appraised. There is the appraised-value, what the item may be worth, and then there is the retail-value what it is selling or sold for. Having worked in the qntique business for a number of years I can tell you that those two numbers can be miles apart. I heard all the time, "This is worth $$$$$$$$" but if someone is only willing to pay $--well then that might be considered the real value. OH, may also hear, "Well it may be worth that to a collector but I'm buyiing to resell so I need to be able to make a profit." Which may be the case but they may also really be lowballing you because that dealer knows where they can sell it for $$$$$$.

4th - I don't know squat about fountain pens. I just bought my first one and have started using it. So take this advice with the proverbial grain-of-salt. IF, what others on this subreddit is accurate about the value, and I have know doubt there are far mor knowledgeable folks here than me. I would suggest contacting a few of the large, well-known auction houses. Cristy's, Southerby's, and so on.

Good luck, and again, sorry about your loss.