what kind of mods are you doing, Are the mods to the frame or the engine. I looks pretty big, could it have come from the steering head bearing area. may be the guy used it as a spacer.
Looks like a home made shim for a motor mount. Never fails, you lose an important shim, so you look for a comparable washer only to find the OD is a tad bigger. At least it was notched, most guys just bend it.