Yes, I had the same problem with an eBay purchase. In my case it got snapped off whilst the seller moved/packed it and they were able to find and forward the missing part.
The missing knob is on a long plastic shaft. On mine it wasn't possible to just glue the shaft as it was quite deformed. I got a small piece of aluminium tubing and used this to join the two parts. You may be able to find a NOS knob matching this, and then piece together an extension to the shaft. I think it just pulls out, so you get get some long nose pliers and pull out the piece you have. You might need a piece of tubing and a short replacement shaft.
Obviously the easiest thing would be to buy a replacement of this plastic shaft, but I could not find this anywhere.
QService are great but I can't see any knobs for these scopes here.
Here's a pic of what it should look like & the part number. Also pic of my broken one.