The original plate was on the LH side of the steering cowl. Probably worn off. Stamped s/n, 'true model' and date code are stamped into RH transmission ledge behind the starter. Date code would be like: 8A16B which translates-8=1978, A=Jan, 16=Day, B=Shift (day). The plate on the B/H. Is the backhoe model indicating length of stick, lever configuration, etc. Type the 19-... number into the New Holland parts model search. All the backhoes used on 550's are 750 models, but the 19-... number is the 'true model' again.
I have a 78 550 and love it. 30" bucket digs like crazy, but can get tippy dumping. Mine has the torque converter transmission and still grunts to a full bucket load when dozing. Just worked on a 575E with 4wd and it didn't seem out push its bucket.
Your fuel problem sounds like the filter in the tank has fallen off and the shutoff has clogged with debris. Look into tank with a flashlight to verify. Fix is remove hood, unbolt tank and lift high enough to remove shutoff (after you have drained the tank of course). You can get an aftermarket shutoff with screen if you want to have on hand.
Most likely hourmeter has been replaced several times during its life, so hours could be anybody's guess.