There have been examples of cutaway revolvers, both for use as salesmen's samples and for training purposes, ever since revolvers were invented. I've never seen an example, but I would guess that Sam Colt had examples to help sell his product.
The first examples shown are from the Tula Arms museum. Close examination of the picture of the 1898 dated example would indicate that a lot of the parts are not original (wrong OTK mark on the hammer, no serial number on the trigger guard, etc.), but it definitely shows the existence of 1898 production.
The second example has later (post 1910) grips and still shows the pre 1928 loading gate key on the frame. There is no obvious Accuracy proof or OTK mark on the front of the frame and there appears to be no Acceptance Commission mark. This leads me to believe that this piece was originally made as a cutaway.
The last example is a post war example probably made for training. This is the most common variety of cutaway encountered, but they are still, at best, uncommon!