Screw jack, mostly called "house jack" around here because that's the most common traditional use for them. The hole below the swivel pad is for a bar to go through, for turning it. When a crew was jacking up a house for foundation repair, moving, or other purposes, some poor soul (or several souls), probably the youngest member(s) of the crew, had the pleasure of crawling under the house and using multiples of these to slowly raise up the house. Properly, you set cribbing well supported under a framing member, jacked up until you could fit another piece of cribbing in, lowered the jack, blocked it up so you could raise the house enough for another piece of cribbing; repeat until (a) you're done or (b) the lunch or end-of-day whistle blows, whichever comes first. My son owns four that I found at a contractor's retirement sale, and uses them regularly when he's doing foundation repair. One of their benefits is that, if you stay within the weight limits, they are not prone to catastrophic failure or bleed-down, issues at times with a hydraulic jack. If the cribbing is done right and placed properly - that is, under the correct framing members - it's not nearly as dangerous as it sounds. The weight limit is generally cast into the base.
I've got a small one that I've used for similar purposes more than once. If you do any heavy work around the house, don't get rid of it.
Wandering...machinists use the identical concept, but in a jack about an inch and a half high, for certain setups on milling machines and other such machinery. I've got one of those from my uncle; can't now recall if I've ever used it.
