Unusual antique metal angle measuring device / rule / gauge - M. Montgomery

I am looking to sell an antique silvered metal measuring tool / gauge that is named M. Montgomery and comes in a hinged fitted case. I cannot find anything similar and it may well be something a workman had specially made as it is personalised. Perhaps you can tell me more about it? If so then please feel free to drop me a message. Heavy for its size and in the case. I cannot find anything similar, so possibly a one off by the person named on it? Looks very old, possibly 19th century or early 20th.

The tool / measuring device has some rust pitting as shown, but has generally been protected inside the fitted case. Case has wear and age tarnishing but still very solid. 

Case measures approx. 7 ¾” high by 5” wide. - tool fits neatly inside of this.

Weighs 845g unpacked. 

This is just basically being sold as you see it in the photos, the photos show the exact condition better than I can explain.

Please note that any international postage will be through eBay global shipping and I have no control over these costs, please note this prior to bidding.