I want a QB78 but on HPA not CO2.I could buy a QB78s for $60 but I will not because I cannot convert it to HPA.
I just checked the end cap from my XS60C. It looks identical to the QB's end cap and threads onto the tube perfectly. They each look like the right-most example in Robert's picture. So if you can find a bulk fill cap for the XS60C, I see no reason it won't work on a QB78.
Got Gen1 XS60C that I swap around the bulk fill cap with older QB78 with no problem but cannot use the bulk fill cap with my QB78S, vent hole is farther back.