I just bought a pair of faux ivory grips for my 1849 from here ...
Once I placed my order, I received an email requesting that I send them a tracing of the grip frame which concerned me a little. After talking to "Don" on the phone, he explained that he had over 90 molds for the 1849 and that "no two grips are alike". My first thought was "then how the heck does the manufacture sell replacement grips, let alone all the after-market offerings?"
Long story short, I agreed to send him a tracing, tore the gun down, made him an exact template, and sent it off in the mail. A couple days later, he called just to let me know he received it and told me that it would take a couple weeks (which I expected).
In the meantime, I had ordered a new backstrap from the manufacture.
It came in a couple days after my conversation with "Don" about the grips.
When I swapped out the backstrap, the screws matched up okay but the backstrap didn't match up properly to the frame.
IMO, this only confirmed what "Don" had told me when he said "no two grips are the same". So, if you do order from a company that claims to have grips for your gun and they do not ask you for a tracing, you may have to do some fitting once you get them. Then again, it may not be an issue for your 1858. But it may be worth asking.