Thanks for all the photos, I can see now exactly what you have.
This is not by the Martin Brothers, but by a nonetheless quite admired and well-collected vintage art potter called Jim Rumph (1942 - 1993) of Santa Monica, California, who specialized in bizarre and humorous character mugs, such as this one, that are certainly reminiscent of a Martin crow. Rumph was tragically killed by a car while walking his bike across the street. He was only 51 and still at the peak of his creative genius, but he created over 300 unique pieces like this one and they range in value from £15 - £3000.
The date you are seeing there is "1973" when this mug was issued, but I can see why you took it for "1913" as the mold impression has made the 7 look very like a 1. The clincher though is the © copyright symbol which was not around in the Martin Brothers' time.
This 1973 Rumph mug is called the "Splatter Bird Tankard", a baby buzzard sitting on the rim of a tree-trunk mug splattering the head of the troll inside with some really nasty pre-digested roadkill.
This original 1973 marked edition of the mug has become quite collectible and has a fairly strong following.
If you were to sell yours on line (eBay, Amazon, etc) it would fetch in the range of £35 - £50.
It therefore has a full retail or replacement value of £100 and is what you should insure it for.
I do hope this helps!
PS. If there's anything more I can help you with on this, please don't hesitate to ask. If not, could you very kindly rate my services (with the stars or "accept" button) as this is how I get paid by JustAnswer at no extra cost to you. We can still continue to communicate here on this thread after you do. And if you have another item you'd like to ask me about, just start a new Question and put "For Robert S....." in the subject line. Thanks, R.