Advice needed on how to proceed with a Cal 321 2915-1

What a wonderful inheritance.
The good news is that the Speedmaster is a very collectible watch and your one, reference 2915-1 is the very first of them (a first edition if you will) and is therefore highly sought after by collectors. The serial number is within range so likely original to the case ( as Achim said, you can apply to Omega for an extract of the Archive to confirm that) and it appears to have its original, and very rare, bracelet and end links (even the spring pins look original!)
The bad news is that the dial and hands have been replaced and the steel bezel is missing. These parts are extremely difficult to find and would carry a high value if you did find them (ie. 10’s of thousands of £/$.)
However if you intend to keep it I would have the movement serviced by an expert in this model (if you give us your location someone can recommend a watchmaker) and leave it in this configuration. After all, it is how your father wore it. You can always try and find the correct parts over time if that matters to you.
The case back is often blank on this model, so that’s not unusual.
Lovely watch, enjoy it!

Put it on ebay. Don't think the big auctions houses will bring you better proceeds given that the most valuable parts are already gone

Are you looking to sell or keep and wear/store away?

A few interesting points to note.

The number on the movement is similar to 2915s I have seen. However the movement itself has characteristics from a later model, and as such we cannot value this movement as an original 2915 movement.

The end links are the extremely rare number six single slot versions and I have seen them sell for over $2500 each. The bracelet may or may not beyond salvation, but even as it is it is probably worth quite a lot, although I won’t put a figure on it because personally I do not value it, but I have seen them sell for between $1000 and $3000 in this condition.

The case is where the value is, and this could be anything from 7000 to $15,000 depending on the condition on close examination. A crazy person might pay more but the vast majority of us would not.

Just so that we can have the point discussed and dismissed, in order to replace the dial and the bezel and the hands in this watch, to bring to original condition, my estimate would be that if you went shopping with $100,000 you would still be looking in 12 months time. I also think that if you did go shopping for the parts, and you actually found a dial, I would be very surprised if it was pleasing.

That gets the idea of value discussed and out of the way.

What I would do with this watch send it to a knowledgeable Speedmaster service person who has access to Omega parts . You don’t want to send it to Omega who would change even more parts. I would consider fitting a metal bezel. These can be obtained from eBay, as original Omega parts, as they were fitted to a recent model and with some adjustment can be made to fit this case. Before fitting I would reduce the outer diameter of this new part and perhaps the side profile, and whilst it will never pass as an original bezel it will have similar dimensions and therefore it will be more aesthetically pleasing then if you left it alone.

You could of course leave the bezel off the watch many people enjoy this look - however I feel the bezel does protect the crystal to some extent .

If I wanted to make this project pay for itself I would simply sell the end links to finance the entire operation!

As Mr Fruit alludes to above, the asymmetric clutch bridge is intriguing. Though the serial is in range, it rather suggests that the movement isn't fully original either.