Part of vintage lens or vintage car or something else?

[Ron (Netherlands)]

This came als a lens cap with the old 12x10" Butcher lens (see other thread about that lens: https://www.largeformatphotography.info/forum/showthread.php?160295-W-Butcher-amp-Son-Rapid-Rectilinear&p=1570081&viewfull=1#post1570081) ...however as you instantly see this is no lens cap, and I'm just wondering what it might be - part of an old car / old jug ?

this is how it came to me:

https://kpmg0072.home.xs4all.nl/Butcher%20lens/lens%20dop%201.jpg

here some close up pictures;
diameter of the big part is about 80mm / 3.25 inch

https://kpmg0072.home.xs4all.nl/Butcher%20lens/lens%20dop%202.jpg

https://kpmg0072.home.xs4all.nl/Butcher%20lens/lens%20dop%204.jpg

https://kpmg0072.home.xs4all.nl/Butcher%20lens/lens%20dop%206.jpg

In the cap at the top under the cover/lid there are 3 tiny screwholes.

[Jeroen]

Lamp fitting perhaps?

[Jody_S]

Front end of a large magic lantern?

[Mark Sawyer]

I'd guess a front aperture for when using the Petzval's rear element alone as a landscape lens, though it doesn't look original.