Sargent 44 lock cylinder

[gregleck]

I recently moved into an old house with a mortise lock on the front door. Key is missing and lock has likely not been changed in 50 plus years.

I removed the cylinder - it is a Sargent and "44" is stamped into the cylinder. I note the "cylinder" has threads at the end but the middle is not cylindrical.

https://ibb.co/McnPFRj
https://ibb.co/XYc0H1W

Can this cylinder be re-keyed?

What is the purpose of the narrow middle part of the cylinder?

[demux]

Can it be rekeyed? Almost certainly, though a pic from the back would allow us to know for sure. Can you get a blank for that keyway to cut new keys on? That depends on the keyway, and I don't have my Ilco reference book handy at the moment so I can't ID it right now (someone else on this forum may likely do so). Worst case though is that you just replace the whole cylinder with another. Mortise cylinders are (mostly) standardized, so you should be able to thread in any other one from anywhere else, and have it work; you should not need to replace the entire mortise lockset here. If you opt for a new cylinder, the two considerations you'll want to pay attention to are that the cam type on the back is the same, and the length is the same. A longer cylinder would also work, though you may need to add some rings on the front in that case to trim it out properly.

[Squelchtone]

The narrow middle is for the lock manufacturer to save money because it means less brass they have to use when they cast that that part and the lock is still functional and secure enough.

[demux]

That depends on the keyway, and I don't have my Ilco reference book handy at the moment so I can't ID it right now (someone else on this forum may likely do so).

The pic isn't perfectly clear, or looking straight into the front, but S4, S3, or S10 all look like they could possibly be this keyway. I doubt you'd have any trouble getting blanks in any of those.