Going from the M. 12/34 and M. 17/30, I'd say that the naming convention at Steyr was M. "year the action was introduced at Steyr"/"year the particular model was introduced". Therefore, an M. 12/34 is a rifle using the large ring Mauser 98 action with a standard fixed magazine, which was first introduced at Steyr in 1912, in a variant/version introduced in 1934. The same would go for the M. 17/30, which would be a M1917 type large ring Mauser 98 action with a Mannlicher clip magazine in a version introduced in 1930.
Had they produced a military carbine with a Mannlicher-Schönauer type action and rotary magazine in 8x57 IS in 1930, I guess it would have been a M. 15/30. Had they produced it in 6.5 Mannlicher-Schönauer, it would have most likely been a M. 1903/30. But that is just a speculation on my part.
And I of course agree that the M. 12/34 was not introduced specifically for Colombia. The fact that in the end it has only been sold there had nothing to do with Steyr's intentions or wishes.
Heavy OT (maybe a Moderator can split this if not desired in here): the M.15 rifle was a Mannlicher type action with Mannlicher type magazine but in 8x57 IS caliber (see Gabriel on page 428). For what I know they didn't want to introduce a new caliber during war, hence it got rejected. Of the M.17 so far no surviving rifle is known, so I'm making "backwards-assumptions" based on M.17/30 rifles that are known and also named as M 17/30 rifles by the Austrian Army documents. As a result to this the rifle would be the same as the M.15, so also a Mannlicher (not Mauser!) turning bolt action but in 8x50R caliber using the Mannlicher clip (so using the M.1895 magazine). The M.17/30 was the same, but for S-Patrone (so 8x56R).
I am unable to tell you where the designation M.12/34 comes from - this is not a designation I had made up. The original Austrian documents speak of a M 34 rifle. Comparing the Luftwaffenkarabiner optically to various rifles Steyr made over the years it however reminds me more of the M.15 rifle than of the Mauser export rifles they made (shape of stock, rear sight, handguard around rear sight, location of sling swivels, front barrel band, etc.). So easier to first find the origin of the M.12/34 designation rather than to make assumptions by optical comparisons..