Welcome to the Forum. The 38 Safety, 4th Model was made only from 1898 to 1907. Almost all were made after 1898, making them "modern" guns as defined by the BATF. Guns above serial #119,900 are subject to all the same regulations as a new one you might have purchased yesterday.
As for the "When the trigger is pulled, it locks up solid", I am not sure what that means? When I say the action locks up solid, to me that means it is working perfectly without any play. Check to make sure the cylinder stop freely pops up when pushed into the frame with your finger.
One thing to try is to remove the cylinder and pull the trigger slowly to see how the cylinder stop in the bottom frame and the hand interact. When at rest, the stop should prevent the cylinder from spinning. If that is not the case, the stop might be badly worn, tampered with, or is sticky/dirty and not freely returning from cycling. Next watch the hand in the rear of the frame that advances the cylinder. When the trigger is pulled through its cycle and held back, both the stop and cylinder should be visible. Neither part should move when the trigger is held back after the trigger is pulled.
Testing this way really helps with being able to see the parts working easily, but in order to perform the test without the cylinder dry fires the gun. I do not make a habit of this, but never had any problems with the firing pin. You can cut a hardwood dowel and fit it over the firing pin hole and have it rest on the rear of the barrel, taped to the top frame if worried about the pin.
I should add that to remove the cylinder, you need to hold the top frame latch up and pull and unscrew the cylinder. The latch acts as the stop for the cylinder's rearward travel, so important to keep the latch up until the cylinder is gone from the quill.
