The H&R Model 1904 is a fairly commonly found item...I have 8 of them in my H&R collection. They were in production from circa 1905 to 1942. Originally intended to replace the H&R American Double Action, they exhibited minor changes to lock work and grip frame and angle. They did not prove to be well accepted, for whatever reason, and the ADA remained in parallel production. They were chambered in 6 shot .32 S&W Long and .38 S&W. All were engineered for smokeless/Nitro powder pressures, so they will shoot modern factory loads if in SAFE condition. The one chambered in .38 will only accept the .38 S&W Cartridge. There are no known upgrades that I am aware of (not even grip panels.) This model has a pull pin cylinder - not swing out, is DA/SA, came in Nickel or Blued finish, with barrel lengths of 2", 4" and 6". Numrich Arms/GPC has spare parts for most H&R firearms. The internals are supposed to be fairly simple, however the frame is solid (no side plate) and reassembly is reputed to be fairly tricky.
The $100 tag may be a bit high, depending upon condition. I would counter offer $75 and settle somewhere in the middle.
Jim Hauff - H&R Collector