Quoted from drsfmd:
I have no idea. How do I determine this? Either way though, I need to sandwich it for the masked layer which would go on the second piece of glass, no? It’s a wood rail, so I don’t have to worry about score reels and credit reels... just backmasking to direct the light to the correct spots. The original had no silver or white on the back. It was screened black in the masked areas.
Your’s looks incredible BTW!
If you have to look through the acetate to see the image, it is printed on the back, also known as second surface. If the clear acetate is behind the printed image, then it is printed on the front, ie: first surface. And see other posts, you don't need or want to put the black out behind the second piece of glass. Layer it right to the back of the acetate. Just don't glue it. Use can use tape or mild adhesive at or near the edges that would be covered by the frame of the back box. If the image is first surface, and you have self adhesive block out, then you can attach the block out right to the acetate.
If you have to do it again, it really pays to use a printer ie: small sign shop, who can print on adhesive sign vinyl. I am really surprised you have one who couldn't. My local Signs by Tomorrow, who is also a friend and fellow Rotarian, can even print directly on glass. I haven't been brave enough to let him try yet.
BTW: the one in the pic isn't mine.