The OED is very wordy and rather obscure (to me at least) about ‘stowe’. It could be the various parts of the frames that shored up the mine and that belonged to the individual miner, who moved them from place to place with him as he worked. But the stowe also acted, when set up on the surface, as an indication of the ownership of the mine (like staking your claim?). That seems to be what is referred to in the quotation.

There is an interesting and dreadful quote given in the OED dating from 1661:

“He that stealeth ore twice is fined, and the third time struck through the hand with a Knife unto the haft into the stowe and is there to stand until death, or loose himself by cutting off his hand.”

Ah, yes, just at the beginning of the reign of Charles II of course, that Merry Monarch!