The Mathamancy Bracer is a Magic Item. It is worn on the forearm, covering the upper part from elbow to wrist. Two straps hold it to the arm. The upper portion is similar to hard plastic and includes a spot where Parson's calculator watch can snap in.Erf-b1-p054 The watch is vital to its function: without the watch, the bracer does nothing.
Sizemore Rockwell hypothesizes that it may be an ArtifactErf-b1-p054. A broad estimate of the value is around 500000 Shmuckers to the right buyer (a Side reliant on Mathamancy), the very value of the Summoning Spell including the casting and support package. Given that the Spell that created the Bracer was created by the residents of the Magic Kingdom, whether the Bracer qualifies by the strict letter of the definition of Artifact depends on the inner workings of the spell itself, something which we have not yet been made privy.
The effects of the bracer, when combined with Parson's calculator watch, effectively turns its user into a Mathamancer.
Predictions of future
The Mathamancy Bracer also seems to be able to make sophisticated predictions about futureErf-b1.5-p039, so the Bracer may have Predictamancy abilities as well. (Or it is just really, Really powerful Mathamancy.)
This definetly pushes the Bracer into Artifact category. It is essentially a Do-It-Yourself-instant-Mathamancer/Predictamancer. I don't think mere mortals have the power to do something like that. (At least as cost-effectively.)