What type of valve has a screwdriver slot and no handle?

Question

This is the shutoff valve for an area of my 1965 hot water baseboard heating system.

What is this type of valve?

Edit 1: The goal is to be able to shut this off but I want to understand what I am working with before I start turning things with potential hot water and boiler-related consequences.

The nut: After using WD-40 I can turn the nut fairly easily but will not do so beyond a very small movement until I have a better idea whether it might suddenly drop out of the valve entirely.

The screw: The screw is stuck, I have not been able to turn it with a careful amount of effort. It looks like it was somewhat mangled in the past.

So I am trying to understand which of these things, the screw or the nut, will shut the valve, and do I need to be careful of the nut coming out entirely.

Answer 1

Usually these are a balancing valve (to balance the flow between loops so that the house heats more-or-less evenly, or at least the fluid flow is more or less even) as another major balance on the heat delivery is typically flaps on the baseboard covers to regulate air flow.

If so, it's generally a flat disc and generally does not "seal" closed, as that's not its job. Its job is to provide a variable resistance from closed (slot 90 degrees to pipe) to open (slot lined up with pipe.) However, one I just tracked down that looks similar does seem to have a ball-element. Here's another with no detail on the type of element.

The nut is a packing nut for the valve stem. If you remove it without draining the system, you will have a leak.

Answer 2

It looks like a screwdriver-turned valve. Probably a ball valve, based on the tangs hanging out that appear to be at 90-degree stops, if there was a handle on the valve.

Web-searching "screwdriver valve" turns up similar valves:

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