Ok finally remembered to look at this. According to my Ideal book (p. 14) the following were standard calibers offered:

1887 (start date of Ideals) until 1897: "500 Express" (I strongly believe this is 500bpe or nitro for black, author gives the following ballistics: 570meters per second, charge with 9 grams of large black powder)

1897 to 1908: "Caliber 50" (maybe 500NE?)

1909: 450NE, "375 English" (2 1/2"), and 303 British (as is shown in the catalogue scans above)

1910: added 405 WCF

1911: 450NE dropped and 8mm Lebel added

1914: outbreak of WWI and double rifle Ideal manufacture stopped

1925: Recommence of manufacture of double rifle Ideals, in 8mm Lebel, 450 NE.

1931: reintroduction of 405WCF

1937: 8mm Lebel replaced by "375 magnum" (I assume the flanged H&H).

1939: outbreak of WWII double rifle manufacturing stopped for the last time.

There were also custom orders for various calibers like: 8mm Lebel, 11mm Gras (the French service cartridge before the 8mm Lebel in the 1870s-80s), 6.5 Daudeteau, 11mm Werndl, 7.92 German, 6.5 Mannicher-Carcano, and 303 British.

NOTE: the author of the book is mostly a shotgun guy, and I have the feeling that some of his names/designations is not entirely accurate, but I am reproducing it "as is".