It is a Delta 1300 series. Readily repairable with a BIG caveat:
You can cause fatal ( to the valve!) damage if you try to unscrew that cartridge retainer nut with too much force. It will twist off some vulnerable tubes further inside. If you take off the round plate, you will see these tubes. First, we can see in the picture that mineral and gunk have affected that screw on nut. Start now applying white vinegar all around the nut. Repeat several times a day for a few days. Then GENTLY try to unscrew, but not too hard.
The most used techiniques to remove the nut are these :
a) Carefully apply heat to the nut with a small torch. This will tend to expand it and allow it to unscrew
b) using a small hacksaw blade held in hand, or a Dremel wheel, slice through the nut, being as careful as possible to not damage the underlying body threads. These threads are not part of a seal, so it is not too critical.
Regardless, you will want to replace the nut, and they are readily available at plumbing supply stores ( not necessarily blue or orange.) Your trim is also readily available, but the antique brass color may not be.
The cartridge you need is RP19804. The limit stop mechanism on the new one has changed from yours, but it is completey the correct replacement part. Just follow the instructions for the limit stop.