[size=12]I don't know the exact range of production for Iron Mountain, but I have seen two sources that help narrow it down a bit.
First, in the blue book (pg 25 of the 5th edition,) there's a typewritten price list that shows "No Name Polished Ware" which is probably the Iron Mountain line. It gives skillets sizes 3-10 + 12 & 14, DOs 7-10, the size 8 Chicken Pan, one plain size 8 trivet, and "Regular Griddles" (probably handle griddles) in sizes 8 and 9. It's dated 5/5/1931.
To see the second, you have to go looking through the old Kettles 'n Cookware newsletters, which are available for members in digital form somewhere on this forum. Volume 7 no. 2 from Mar-Apr 1998 shows on pg 19 an illustrated advertisement that has all of the above, in addition to long griddles in sizes 7-11, optional trivets for all DO sizes, and optional skillet covers in sizes 7-10 only. The ad has Griswold's name and logo, and refers to the line by the "Iron Mountain" name. The KnC newsletter dates that ad to 1923.
So, provided that all dates are accurate, they were at least in existence as a full line by the early 1920's, and they were starting to be tapered off by the very early 1930s.[/size]