Speed Six-Fixed sights, round butt, 2 3/4" barrel.
Security Six- Adjustable sights, square butt, 2 3/4", 4", and I believe a 6" was also available in barrel lengths.
Service Six-Fixed sights, square butt, and I'm not sure on barrel lengths for this one, I believe it was only 4" and 6".
Good guns though, smaller than the current GP series from Ruger, about the size of S&W K-frame, maybe slightly larger. They do handle magnum loads a little better than the K-frame though, and they are usually cheaper than Smiths as well. I recently got a LNIB 4" blued security six for a hair under $300, and a NIB stainless security six 2 3/4" for about $350.
If you're thinking about getting one, and see one at a good price, you won't be disappointed.