Agree they use different seals and washers. I had an O2 reg spit a little Teflon type seal, not a thing wrong with the diaphragm.
But I have a cheaper one I put on the reel circuit and bought as needed from HD, was the perfect size and it cost like 40 at the time for a set. I had a Sears lasted about 20 yrs and finally broke a part I didn't want to fuss with. Same for a lot of used equipment, I will take the thing apart fuss with it, make or find a part if its fast and easy or I can do it immediately etc and I aint above buying a new one to get going again. No point in waiting for 20$ in parts and you still got to fix it.
In this class it absolutely aint got to be hi dollar or badd ***. I pipe what I can with 1/2 and the 3/8 I can do as I got a lot of fittings. Lets one bush a single step, use ******* and elbows/t's bit better than 1/4, a turn or 2 doesn't add much resistance and they reg has 1/4 ports also.