I generally agree with you & Chad, but here's my thought process:
Zip codes started in 1963, and postal zones (one or two digit codes) go back to 1943. This one has neither, but it certainly doesn't pre-date 1943, so the absence of a code doesn't necessarily date it. (I've found the presence of a zip code or postal zone to be good at dating a furnace, but the absence of one doesn't mean much. It could be that they didn't feel the need to add a zip code (or postal zone) since they weren't mailing it. . . )
I agree about the chrome and the likelihood of it being from the '50s, but the sticking point is that this was a gas furnace from the get-go - it wasn't converted from oil, and there's only a small chance that this house had gas service in the '50s. Portland didn't have any natural gas until 1956, and even then, it was quite rare until the '60s. (We had manufactured gas much earlier, but that was long gone by the '50s.) So the '50s is possible, but unlikely.
The data plate states 66% efficiency (90/135), which is probably what it gets when everything is perfectly balanced, it gets a rolling start, and it has a tailwind behind it. One of my partners used to do combustion analysis on these things and he said that, once tuned up properly (which wasn't particularly difficult), they could deliver about 65% efficiency pretty reliably.
I was able to get a good view of the burner and the outside of the drum-style heat exchanger, both of which looked great. In the report I observed that the furnace is old and inefficient, but paid for and that it would probably outlast several
If anyone's interested, I included this paragraph in the report: