Without a built-in light meter, it doesn't matter. Many 35mm cameras had a small slot in back of the film door as the "memo holder" (you tore off the box top from the film box and slid it into the holder -- your Pentax K1000 has one of these). That was just so that YOU remember what film was in the camera.) It does not actually change how the camera behaves. So long as you remember the ASA (the equivalent of what we now refer to as ISO), that's all that matters.
You need to know the ISO because when you use a light meter, you have to set the meter to match the film ... otherwise you wont get valid exposure recommendations.
On a budget, a Sekonic L-208 would be a fairly basic but effective light meter and pretty much what would have been used back when that was a current camera (not that the model existed.. rather that the capabilities of this meter match what a photographer would have used back then.) It's about $125. This is probably the least expensive meter that would get the job done.
The meter is very basic... set the ISO, push a button to take a meter reading and then rotate a dial to match the arrow to the needle.
There are two disks in the middle of the meter... one has shutter speeds, the other has aperture settings. As you rotate the dial to align the arrow with the needle position, the dials align to show you all the combinations of shutter speeds & aperture combinations that are valid for the amount of light you measured (at the ISO you set).
It can measure either "incident" light or "reflected" light. "reflected" light is what a modern camera's built-in meter uses... the light reflecting off whatever subject you pointed it at. An "incident" reading is technically more accurate because it measure the amount of light hitting the meter from ambient sources and is not influenced by how reflective your subject is. But to take an incident reading, you have to hold the meter where your SUBJECT is located (not where your camera is located) when taking the reading.
It cannot meter "flash".
The L-308 is a step up because it can can also take flash readings. When you put it in the flash mode, it's reading the light continuously and waiting for a big spike of light to hit the meter and it saves the peak light that it read (the light from the flash). This means it can be used to find a good exposure if you plan to shoot with a flash.
There are higher-end meters that get fairly sophisticated... and more expensive. They can deal with flash ratios, % flash contribution (vs. ambient light), exposure averaging, measure dynamic range, have radio controls to trigger (and sometimes even set) remote flashes, etc.
