I would say it's a rebuild. I've read that Singer authorized certain repair facilities to rebuild machines during and just after WW II. Those machines were painted with the black crinkle paint.
What does look odd is the silver paint on the bottom. But I've never had a rebuilt machine to mess with, only a factory crinkle 66-18.
I'd pull that machine out of that non Singer box and flood it with Tri-Flow oil, after taking all the plates and hand wheel off of it.
The bottom pieces will need coaxing but I'll bet they free up pretty quick.
Oh, if you trash that case, please save me those two hinges. My Alden's case has a broken hinge and they are all but impossible to find.
Joe