There was an old Cast Iron newsletter published in the late '90s and early 21st called "Kettles n' Cookware." It's available as a download somewhere on the members' side of this site, and I'm pretty sure that it was put out by one of the two authors of the blue book. In the Mar-Apr. '98 edition, there is a reproduced Iron Mountain ad dated to 1923. It's a little more complete than the list given in the blue book, but then again the line probably got bigger or smaller with time. It lists skillets in sizes 3-10 inclusive + 12 and 14, Handle Griddles in sizes 8 and 9, Long griddles in sizes 7-11 inclusive, Dutch Ovens in sizes 7-10 inclusive, and the covered Chicken Pan. As far as rarity goes, I think that the low numbered skillets are fairly common (except perhaps the 4,) but the 14 is a little tough to land. I haven't come across too many of the long griddles -- the blue book lists a 7 and an 8, I think, but I did see a badly pitted 9 once, so I guess that they trimmed the roster at some point. I haven't ever seen a 7 or a 10 DO myself, but I know that others have. I think that the two handle griddles aren't very rare. The chicken pan is common.
I know that they also made skillet covers in this series, but I don't know how many sizes, and I've only seen pictures. They're less common, I'd say. They do exists, but they aren't listed in the aforementioned ad reproduction. They are in the blue book p/n appendix. The high-dome cover that was made for the chicken pan was NOT a part of the IM skillet cover line, by the way. I think that the IM skillet covers were all low dome.
Some people group the 754-755-756 Victor-like unmarked skillets into the Iron Mountain family, but I don't personally agree with this.
Speaking only for myself, so far I've been able to obtain skillets 3-8, a size 8 handle griddle, a size 7 long griddle, size 8 and 9 DOs, and the cover (but not the skillet, darnit) of the Chicken Pan. I've seen 10 and 12 skillets, but none have ever come my way. I've never seen a 9, but for some strange reason I just have a hard time finding size 9 skillets in some of these series, and I've never seen a 14, either.