Actually Prentiss had three lines:
1 - Machinists' Adjustable Jaw (Top of the line, the static jaw had the pin you could pull out to swivel)
2 - Machinists' Bull Dog (Solid Jaw)
2 - Monarch (Solid Jaw - "Lighter Duty")
I think my old scans of the Monarch line are on my other computer at home. I *think* they might weigh a little less? I haven't heard anyone complain their Monarch was too wimpy, so who knows...

I do like the lion logo, that is not something you see on vises of those days, most just were pretty blah with their name, model, and patent info.
EDIT - Quick ebay search I found some pages...
To compare some of the same sizes:
4.5" Jaw / 6" Open / Stationary
Adjustable Jaw (No. 3) - 54 lbs
Bulldog (No 52) - 52 lbs
Monarch (No 217) - 42 lbs
Although I found in a different print the No 52 weight changed to 42 lbs... So I don't know if one is a mis-print or what...
I'm pretty sure I read on one of the old prints they said the monarch was lighter duty, but I could be wrong. Someone else posted that there was actually a Monarch company and simply that Prentiss made the vises for them.
Either way, bulldog or monarch they are both sweet...
