About that legend of General Custer carrying one of the big double blades. Given Maher and Grosh was founded in 1875 and Custer died in mid-1876, that doesn't give him much time to buy and carry their knife. I suppose he might have been using a similar knife from another company and M&G decided to copy it when they found out. The catalogs never gave any real details on their statement.
As for the El Diablo model, their 1908 catalog gives a short story about a knife fight between two Mexicans with them. Not exactly what I would consider a folding fighter, but I suppose you use what you have.