Thanks a bunch guys. I need to get a look at this gun. From what I've gathered, pre 1963 37's are a pain to swap the barrel on. So, if its really old and rifled, I may have to pass.
The Pre-63's were not designed with interchangable barrels. It's a pain because to change barrels, you have to have the old barrel removed, and the gun has to be retrofitted to accept new interchangable barrels. This can get pricey - but it is a 1 time cost.
You should be able to tell straight away by the serial number. It's not that 1963 is the magic cut-off year, but rather anything with a serial number BELOW 855,000 is non-interchangable. Anything above is interchangable. Just so happens that serial came up in '63.
One other thing to be aware of - is because of the technology at the time, just because the barrel is interchangable, does not mean it will be 100% easy. Tolerances have gotten a lot tighter and production more consistent over the years - so if you take an old (say '64) M 37, and try to put a new barrel on it - it might be tight going in, and take a bit of patience and/or work to get it on. They'll still fit - it just might be a tight fit the first time you try to put a new barrel on. Best bet is to start to put it on, as it gets too tight, take off again, start over - repeat.
In some rare cases - you will not be able to get a new barrel on an old gun - but this is somewhat rare. The closer in age difference between the barrel and the gun, the less likely this is to be. Newer guns are also less likely to have this problem.
Aside from that - one of the things that i think makes this a particularly capable HD gun - is the "fanfire" capability. Basically - after you take the first shot, hold the trigger back, and as soon as you lock the pump forward, it will fire again. As fast as you can work the pump while holding the trigger, it will spit out shells. Part of the reason why some LE still employ it for "riot gun" duty. Long distance or precision work might not be so great with this, but in short distance "drop the bad guy" situations - that's a pretty nice feature to have.
If you're looking for an HD shotgun - it is a good choice. Even if this one you're talking about doesn't work for whatever reason, check gun broker and local shops. There's a few variants, including the police model, out there - that will absolutely make for great HD.