The author is referring to a criminal gang that manufactures counterfeit coins. You make such by pressing a blank metal disc between two engraved metal plates bearing the pattern of the coin. These are the dies (s die) AskOxford: die<sup>2</sup>. The penalty for those caught in the manufacture would be greater than that for those involved in the distribution network.These were coiners of money, and they made some very good offers to me, as to profit; but the part they would have had me have embarked in was the most dangerous part. I mean that of the very working the die, as they call it, which, had I been taken, had been certain death, and that at a stake
BTW. This was written in 1722, much earlier than Victorian English.