I began this post with the intent of asking for assistance in identifying different papers and/or gums used in the production of the Mozambique Company coat of arms REPUBLICA overprint set (Scott #75B to #89) from 1911. Before finishing, I discovered at least a partial explanation for the differences in paper/gum color I am seeing in the image below.
Thinking this information might be helpful to others, I decided to go ahead and present what I've found and let others fill in the blanks.
I had previously scanned six values from my set of the 1911 Lisbon REPUBLICA overprint set and prepared this image. It shows at least two, and maybe three, different paper/gum types. The first five stamps in this image are Scott #75B to #79 and the sixth is #88. As you can clearly see in the scan, paper/gum colors differ.
My 2016 Scott Classic Specialized Catalog doesn't mention reprints or different papers or gums for the set. However, I once belonged to the International Society for Portuguese Philately (ISPP) for a short time and still have my Portu-Info issues from that period. In issue No. 133 (July-September 1999), page 100, editor John Cross mentions a reprint of the 10r green (Scott #77 yellow green) and explains it was "...'reprinted' at the Casa da Moeda where the stamps, supposedly, were immediately overprinted with the 'Lisbon' REPUBLICA." John goes on to mention three papers that were used for the reprints: "porcelana ordinario, esmalte and lozenge (with vertical lozenges)."
That paragraph from ISPP does not mention the white gum that I see on my copy of the 10r (and 5r and possibly the 500r). All of the other stamps in my set (not shown) have the yellow back. I also checked my set that was locally overprinted (Scott #61A to #75) and all of them clearly have the yellowish back except the 500r (Scott #74). The back of my copy of #74 has the same color as the #88 stamp in my scan.
After finding the ISPP information and searching online, I discovered that the same ISPP issue referenced above includes an article by John Cross regarding the 1917 Red Cross issue. John included paper descriptions as well as a table showing which values were printed on which papers. He also provided hints for differentiating the paper types. Along with other paper details, John describes the back of the porcelana ordinario paper as yellowish. The esmalte paper is shiny, but without the coating that is used on the porcelana and porcelana ordinario papers. The pontinhado em losengos (lozenge) paper has vertical lozenges and is more white. He also identifies a porcelana (porcelain) paper and finds it difficult to differentiate it and esmalte paper, but does not mention the color of the backs of these papers. He offers a clue though. Among the Red Cross overprints, the 100r and 700r were printed only on porcelana paper.
My 100r from the Red Cross set has a back side color that is midway between the white and yellowish (2½r, 15r and 20r) stamps shown in my scan. The other clue that John offers is the fact that the esmalte and porcelain papers appear uniform from the back.
I can see no evidence of the lozenges in the stamps I have looked at, but John's table for the Red Cross issue indicates that was the least common paper used for that set of overprinted stamps. The same may be true for the 1911 Lisbon REPUBLICA set. ISPP probably has published a similar table of the paper types used for this 1911 set, but I haven't found it in my small collection of Portu-Info issues. My bottom line is that my 5r and 10r must be reprints and that was what I wanted to know in the first place. Getting down to the paper type for these stamps wasn't a concern and would take more time than I have available. Hopefully, this information I'm presenting will be helpful to others who find the coat of arms issues a bit perplexing!