Mike;
I have made hammers for Grant SLE guns and I make them from O-1 alloy steel. As a matter a fact I have a 1889 Grant SLE that I made the left side hammer (actually called tumbler), and the bridle and as you suspect it is a difficult and long process. Making a SLE tumbler and getting it to fit properly against the side lock plate and tumbler is the most difficult task many gunsmiths will ever encounter. To make a proper SLE tumbler one first must make jig/fixture to be able to create the tumbler center post that is dead center to each side of the tumbler---that is the problem that causes most gunsmiths the most problems.
However, I still have in my Grant parts box the original left side tumbler for my 1889 Grant, which was not the original to gun but one that some English gunsmith had installed. It was not the proper one for a 1889 Grant as in that time frame the tumbers had an extension at the base that activated the ejector system (series 1 and 2 Stephen Grant patent ejector). I suspect this Grant tumbler will work for your gun
Send me a PM with your email address and your cell phone and I will get up with you and discuss the path forward. I am in Blythewood, SC which is 18 miles north of Columbia, SC. Via email I will send you a photo of the left side lock tumbler, bridle, sear, spring and so forth to compare to yours.
Kind Regards;
Stephen Howell
