... I think it too should have the check valve. On both of our 1980 W123(s) we found this single check valve on the Door Lock System under the dash up high just above the steering column where the yellow Door Lock System solid Yellow vacuum line turns toward the driver's door side. I just checked my favorite on-line parts source [because of their graphical part blow up interface] and for your car they show such a check valve although theirs seems to be in the engine compartment. They also show it to be blue & white plastic colors. I don't remember what color mine is.
Take another look under the dash and trace the hard solid yellow plastic vacuum line as it passes through and turns toward the driver's door and I think you should find it.

What are your symptoms? I'm doubtful this check valve will prove to be your problem unless someone has pulled it loose. This is a protected area so I think it should still be in good condition... and there are so many other more likely problem areas... especially the reservoir tank in the trunk and the many 4-way and 3-way rubber connectors.

I eventually want to put together a page or two of text describing these trouble areas and innovative fixes!
Regards