So what does the "X" stand for??
There are some on here that can give you a pretty good historical of Parker vises, I have some limited info.
Their first vises were around 1860's and were oddball models and sizes....like 0000, 1,2 and other random models
Somewhere just before or around the turn of the century they came out with the Eclipse & Superior series.
The first gen Eclipse were the 100 series for fixed bas & 200 series for swivel, then later replaced with the 2nd gen which were 950 for fixed and 970 for swivel.
The superior 2nd gen were the 800 series, the 803, 805...being fixed and the same in the 820's series like 823 & 825 being swivel.
The superior 1st gen were these X models. All of the 29x, 39x, 49x and so on were fixed, the same model starting with a 2 like 229x, 239x, 249x were the same but with a swivel.
The Eclipse were great vises but the Superior were considered the super duty model, so Joe may be correct, the "X" might have meant heavy duty.