I recently bought a job lot of MGB parts from eBay and amongst it was this item which i couldn’t initially identify. After posting a picture of it onto the mgoc members forum i soon got a response from one of the more knowledgeable members with the answer.
It’s an aftermarket accessory (part no. RHD R248 MGOC) that fits under the drivers side dash to hide the wiper motor and all the wiring in that area. I guess it looks neater and it makes getting at the wiring of any potential thief a little more difficult.
It utilised an existing hole in the firewall for the back of the cover but had to drill a couple of small holes for self tappers for the front of the cover.
A PO had drilled a large hole in it which i had to modify again to fit around the more later collapsible steering column.