I got this piece....?

[Macguyver]

What you have there...

I could be wrong, but judging by the way it has a recessed groove around the outside to mate up to a seal, and the honeycombing to increase strength against deformation from pressure differentials, combined with the two legs which look to be used to keep something spaced at a set distance or pressure, and the accumulated gunk around the honeycombing pattern, but not in the center of them, I would say it is an airbox cover. Likely from another motorcycle, and not yours.

Perhaps your bike had a torrid affair with a honda and kept the piece as a momento?

*EDIT* Upon running a search for Toyo Roki Seizo, the only relevant thing that came up was a vague reference for an air filter patent.

Did the PO have any other bikes kicking around when you bought it? Maybe you can get a hold of them and let them know you will return their airbox cover for the paultry sum of... one billion dollars.

[Griffin]

Could indeed be an airbox cover, or.....

Some GS models have a plastic piece that fits over the battery and under the seat. Does it look like it might fit there? The purpose of such a piece is to prevent the seat pan from touching the two battery terminals and heating up your posterior.

[chef1366]

Looks like the airbox cover on the 450 I worked on.
Good call

[gane]

Is bit small enough to be a cover for throttle cble linkage?

[Calvin Blackmore]

it does not look much like it but their is a plastic bit that covers frame by the steering stem
purely cosmetic though

could that be it

fiche makes it lok like a left side only part

[chef1366]

It is smaller than a bread box and also an airbox lid.