That's a terrific find! Congrats.
As for the smooth finish plastic, it seems typical of prototypes/pre-production machines.
An eBay auction for a prototype Macintosh SE explains:
"If you collect Macs, this 'smooth finish' instantly sets this Mac apart. It reflects light and shines, as you pass it.
It is hard to see in the pictures, but the finish is mirror like , to the touch.
I wiped the panel with a damp cloth to try and highlight this smooth finish.
Such parts are usually the result of test runs, on the new mold/tool.
Usually only a few hundred test pieces are made.
When the molded part has passed inspection the final treatment of the mold's surface is applied to give the textured finish we are so familiar with."
Source:
https://www.ebay.com/itm/Apple-factory-Macintosh-SE-with-Prototype-smooth-finish-case/173874404360
