The casebacks of all Universal Geneve “Nina Rindt” are flat, with stamping/engraving, which are often worn and can be difficult to read.
There are two exterior caseback designs for the UG 885103/02. Both versions have the reference number (as 885103/02), with the serial number below at the “base”.
These is a very wide range of stamping/etching appearance for the reference and serial number. Some I believe were likely etched later and are not original to the watch, and perhaps the same for some stamping. Some will have a mark (‘) to break up the serial number (as with 2’453’303), and some will not (2569258).
Some casebacks feature the text STAINLESS STEEL (top line) and SWISS (just below), centered at the “top” of the back of the case (in line above the reference and serial). We see pieces both with and without this text within the same range, and I haven’t been able to determine when it appears.
I have not seen a relationship between when the mark is included within the serial number, as discussed above, and when the STAINLESS STEEL SWISS text is included. Do not read into the below that STAINLESS STEEL SWISS text always has serial numbers with an apostrophe, or that those without the STAINLESS STEEL SWISS text have serials without an apostrophe.
The inside caseback design is the same across all pieces. They are stamped with, horizontally aligned: the Universal Geneve shield logo (near the top), empty space in the center of the back, then ‘ACIER INOXYDABLE’ (small text), ‘SWISS’ (larger text), and finally the Huguenin Freres logo. The lowest serials (2.41) seem to have somewhat distinctive concentric circles within the caseback as well.