You can see for yourself that the grips of the two guns are not the same. As you say, the grips for the 1892, etc. are available. I have used grips from this company and found they are OK and good repros. Further, they are plastic rather than hard rubber and won't break or chip. Here is the page with the 1892/94 grips:
http://www.gungrip.com/items_22__page2.html
But they don't list the Model 1889, probably due to lack of demand since the gun had low production.
I don't know where those grips came from on the 1889. My best guess is that the gun was updated around 1908, and the Colt factory put on grips of the style then current, along with that type ejector rod head.
Gun Parts Corp has some parts for those guns, but they are scarce. The mechanism is unlike any other Colts, either before or after them. Common problems are broken hand and cylinder lock spring; they can be made but are not usually available.
Hi, Spacecoast,
FWIW, after about s/n 200,000 and with the Army Model 1903, Colt began to bore the chambers all the way through for use with the .38 Special. The usual advice about the guns not being strong enough does not appear to be correct, as the cylinder is the same diameter and material as that of the Army Special, which was made for the .38 Special. Those late guns, though, will chamber .357 Magnum, and that is definitely
not to be fired in them. Of course, the .38 Special is the loading of the day, the standard 158 gr roundnose bullet, not +P or +P+ loads.
Jim