I would imagine the ductile iron would be heavier, as it normally is but I put these vices side by side and the cast steel one as more meat on it on the fixed jaw and around the anvil ,they are both well made ....you are correct concerning the p ,I read it was just ground off.Realistically, there's no functional difference between the cast steel versions and the later SG Iron versions. I'd keep whichever feels nicest.
Just out of interest, do they weigh the same?
I had a feeling the "P" was present in the casting pattern for the dynamic jaw and was therefore present on all the castings. It just got ground off the castings that were used for the quick-action versions.
Their is some numbers under the steel one.... one is 112 obviously the model ,the other says 35 could that be year of manufacture or batch number ?