Appears to be open car front seat riser, but need measurements, width and length. And more photos from all angles. We can figure this out, but need more info.
The seat frame looks to me like it is for 28/29 roadster. It appears it is a reproduction as I see no mounting holes in the flanges at the bottom to mount it. Original seat frames are riveted together with braizer head semi-tubular rivets. This frame looks like it is spot welded like most repro frames.