Yes,it is a needle feed that will sew 5/16" using 138 thread max,your holsters would have to be on the very thin side for this machine to sew them.This model is for thinner leathers for sewing wallets,credit card holders.
As far as it's backed off I really doubt the tension is the problem,my guess is look for it to be binding ( or getting caught) somewhere between the spool & machine.
@Ken1967
The manual doesn't show threading just some adjustment tips. Here's a pic of one threaded. You go through the tension & then take off the brass piece holding the rubber stripper (at the bottom of the lube pot) run your thread though the rubber & then to the thread throw piece by the needle.
If your bobbin thread is showing on top it has nothing with the #23 needles,we have people using that size all the time.I would suspect you need to tighten your bobbincase tension spring to get it to pull down.If that doesn't work you should get a new bobbincase spring.
These have been discontinued many yrs ago,I sure hope you can find one as I have 5 machines here that need them ,I've been looking for at least 10 yrs.
needs a plate on top to enlargen the table though, because the "footprint" of the 29K-33 is a lot larger than that of the 29K-51.
You can use 2-pieces of angle iron,I use old bed frame cut the length of the legs to extend the mounting surface on the short top.
Look @ the 190 system of needles it measures 1.98mm shank & 44.50 mm from top of needle to top of the eye hole making it approx 6mm longer than a 135x16