From ErfWiki

Proposed Canon

Mathamancy Bracer is a Magic Item, possibly an Artifact^Erf-b1-p054, that Parson received in his second Stupid Meal^Erf-b1-p053.

It looks like a sleeve of hard plastic, but has a spot into where Parson's calculator watch can snap.^Erf-b1-p054

The bracer instantly and effortlessly does raw calculations necessary to analyze probabilities and predict outcomes of battles - that is, Mathamancy.^Erf-b1-p054

Speculation

The effects of the bracer virtually, but not technically, turn its user into a Mathamancer.

Apparently it is an item of great interest to Mathamancy dependent factions^Erf-b1-p060 like Charlescomm^{Erf-b1-p103Erf-b1-p105}.