All radiators are heating except one, so I'm trying to bleed it. There doesn't seem to be an obvious bleed valve (unless it's that nondescript, frozen, simple bolt on the top front corner of the left end of the radiator). I'm trying to determine whether my system is steam or hot water. I have read/viewed multiple online articles and videos, but this all seems inconclusive to me.
It's a two-pipe setup, with the apparent inlet at the top of the left end (this pipe is warm) and a valve at the bottom of the right end (this valve and pipe are cool). Pipe at top would suggest hot water (right?), but a plumber once told me my system is steam. I take statements like this with a grain of salt, however, since a contractor also once told me my boiler runs on heating oil when I know full well it's natural gas. Furthermore, the internet seems to say that a steam system's radiators will have air vents halfway up on one end of each, yet mine do not; and the valve styles are mixed (some are cylindrical while others are globe-shaped).
Since the radiator characteristics have not definitively answered this question, I also looked at my boiler. Here the apparent lack of a sight-glass and the presence of an expansion tank seem to suggest that it is — or at least was — a hot-water system. But if this truly is HW, then where are my bleed valves? And why did the plumber who once did work on the system tell me it was steam?
I think the boiler was originally oil but was converted to NG, so maybe the radiator system was converted from HW to steam? The house was built in 1920 and the boiler is old as the hills. Any help untangling this mystery is very much appreciated.
EDIT TO ADD: Here are a few close-ups.
More pictures, January 23: