Since you have an older, pre WWII, model I strongly suggest that the compression chamber will need to be pulled and modified. The above drawing shows this modification. The earlier compression chamber does not have the groove cut for an O-ring. These compression chambers invariably start leaking at some point. This leakage, reduces the pressure that the gun will be able to hold.
You pull the compression chamber by pushing it out from the muzzle end of the compression tube. My tool for this is an old 101 barrel. It usually only takes a few hits with just the weight of the barrel to get the compression chamber to pop out. Be sure that the trigger sear is not in the way though.
Once the chamber is out, it only takes a few minutes to cut the groove with a lath and then reinstall with an O-ring.