Hi HW......welcome to TFF.
Well, the first thing you need to determine is whether the engraving was done at the factory or after the pistol was purchased.
I'd suggest that you get a Colt Historical Letter (http://www.coltsmfg.com/cmci/historical.asp
This will tell you how the gun was when it left the factory, and who the first purchaser was. If it was a factory engraved gun, perhaps they can also tell you who the engraver was.
Also, if it was engraved at the factory, there's a book.....Colt Engraving by R.L. Wilson, that might give you the info you want. It's a two volume set, and very expensive (http://www.wilsonbooks.com/ColtEngr.html)
, but perhaps a library near you might have some copies.
If it was engraved after the factory.......well, that's going to be a lot tougher.
If the first purchaser (the Historical Letter will tell you who that was) was a large wholesaler or large gunshop, perhaps they had some in-house engravers who did the work. If they're still in business, perhaps they can help you there.
If it was done by a small local craftsman.....well, that's going pretty hard to track down.
BTW, just in case you didn't know, your SAA was made in 1906.