Not particularly rare in the UK, if its in original paint though that is rare. Usually they are just rust coloured ;)
I have an early one that appears to be in original paint, however it is grey. No idea when they they changed from grey to red, or if the grey on mine is original or just a very...
I wonder if the big flat release bar has been put in incorrectly, if the screw was removed and the bar rotated 180 degrees and refitted it looks like it would run in a straight line then. (although removing the screw may be easier to say than do ;) )
I'm not quite sure which bit you think is...
The later ones were blue, didn't say auto vice on the side, and didn't have the valve holes.
Somewhere in this thread is a post or two showing all the variants of the Record 75, its likely the 74 went through the same changes.
This is interesting and supports your doubts on the weight.
1906 article on the vise, 2nd page shows the usual advert but states 385 lb
https://archive.org/download/PittsburghAutomaticViseToolCo1906/Pittsburgh Automatic Vise Tool Co 1906.pdf
1907 article on the pittsburgh vise and on the 6th...
This is the only picture I can find at the moment, unfortunately it's of the swivel jaw variant.
Like you I doubt the massive one was ever made, probably got no orders for it. If it had been made surely it wouldn't have been scrapped, or at least a picture would survive.
There are smaller versions of that style kicking around, much smaller.
Yank vises do seem to have a lot of weight to them, at least according to their catalogues anyway.