Overall dimensions of metal clarinet barrels vary considerably. For example length can vary from 40mm to 67mm depending on the instrument, and that doesn't even take into account that some barrels have tenons and some have sockets. I think measuring your other clarinet and going from there is a great starting point.

They aren't too uncommon, you can usually find at least one on eBay at any time. Obviously this one is overpriced but it looks nice and even comes with a wooden Buffet mouthpiece!

https://www.ebay.com/itm/143075938993?hash=item214ffd92b1:g:OjUAAOSwVWpa-Eh~

Are you just collecting these for fun or do you actually play in a HP band?

Saxophones have huge tone holes. Even on Sopranino sax the lowest tone holes would be too large to cover. Also on most sizes of sax the fingers are not always directly above a tone hole meaning the instrument would need a complete redesign.

The closest instrument to an open hole sax would be a Boehm system tarogato.

You need to go through all of the pads with a feeler gauge. A leak light can help but it won't tell you the whole story.

Here's how to make one, though I suggest using a plastic shopping bag instead of the tape as it will be more precise.

https://www.youtube.com/watch?v=Xg1DJhEBZEM

Some pads to pay close attention to are the Ab and the Ab/Eb keys, they are some of the harder ones to get level.

Seems like a spring issue. One of your keys is opening slightly when you play causing the fluttering sound. Most likely this would be the F#/C# key but could be any key held down under spring tension. Try pushing the pad cup down somehow while playing and see if it goes away.

-Jdbassplayer

I'll order the next size up, 1/16" thick instead of the 3/64 as mentioned above.
Cheers!

Why? That's just a waste of time and money...

Let me clarify how the key work functions. The lower joint is standard Boehm system. The only difference is that there is no linkage to the upper joint as there is no need for an alternate Eb. For the upper joint, Eb is fingered xoo|ooo. E can either be fingered oxo|ooo or by playing an Eb and using the first trill key or the sliver key. F is normal but F# can either be played with the first or second finger.

The plate with the vent hole is there to help with the altissimo like on bass clarinet. This is needed because the first finger tone hole is now a semi tone lower, meaning it is less effective to just lift the index finger. The plate is connected to the third finger ring so it will close automatically when you play an altissimo note.

Carl Fischer has been around since 1872, so 1896 is a perfectly reasonable date and I would see no reason to doubt it. Looks to be in very nice shape!