Found original part?

[savagecreature]

More information (sorta) on this piece.  (Maybe this sorta stuff should go on the wiki?)

I did some research. As Lichtbringer has already told us this part comes from the Paterson Trident hand held slide viewer.  It's an inside secondary lens that is accessible when  you open the back/bottom of the viewer to access the battery compartment

You can see the lens in question in place there just below the hinges. 

Paterson also sold another model of slide viewer, the Viscount. It seems a strange coincidence that it would share the name as the plane the Astromech Holo Projectors come from.  The Viscount looks, at least at first glance, identical to the Trident with the exception of being molded it green.

These Viscounts are more common, at least here in the states, and can be had for fewer dollars than you usually pay for a Trident.

They aren't, however, as similar to the Trident as they seem. First and foremost, they don't have the inner lens we're interested in for the Treadwell.

A printed insert that comes with the Viscount does refer to an inner lens for increased magnification that comes in the "Special Model" but it is unclear if the lens is the one in question or not. The description of their being two grooves that the lens slips into implies not.  You can click on the above image for a larger version where you can faintly see the engraved positions the text refers to and the grooves on side of the inside of the shell.  It looks like this additional lens serves the same purpose as the lens we're looking for, it's just implemented differently in the Viscount than it is in the Trident.

In addition, for someone planning on building an accurate version of the Detention Block camera array, there are differences as well.  The Trident has a curve to the lip extending in front of the main viewing lens, the Viscount's is straight, and there are two straight flanges on the bottom of the Trident that the Viscount lacks. It actually appears that those flanges are directly related to our treadwell eye lens on the inside of the device.  And most importantly it appears that the Viscount has a smaller front lens, which would change the look significantly.  

All of the Trident images in this post are now in the reference gallery. All of the Viscount images including the complete text of the printed insert (as I didn't want to include incorrect reference in the treadwell album) are in my personal gallery in the Random Stuff album.