I hav something similar, made by a company called Alloy Products. I have a 20L and ~12L one, there are not actually couplings welded in, more like weldless bulkhead fittings. In any case, you need to affix one of these gas connects
and one of these liquid connects
to the ports. The liquid one will go on the diptube.....the gas on the opposite port....then seal the other 2 ports or add a thermometer for kicks. I'd like to find the part that will adapt these as well, because I have 2 unuseable kegs right now.
EDIT: This PLACE
probably has what you're looking for. I might have to hit them up myself