Yep, should be horn. Loads of those older german doubles used horn for both the trigger guards and the buttplates. They are interesting, but prone to breakage if one isn't careful. If you do break the horn trigger guard, be prepared for some hand wringing as they just aren't available anymore. You are correct with 70 mm being 2 3/4". Lots of the older german ones are marked 65mm. Far as value, I'd need to see a picture and can give you an idea. Another source of value is go to Gunbroker and plug in Sauer sxs. Some for sure will pop up. PS. I've found horn trigger guards generally don't help the value because of the breakage issue.
