Bavaria Retourbriefe (Returned Mail) Forgery Identification?

I have a set of six Bavarian "Retourbriefe" stamps, which were really more like official seals than anything else... they were used to seal an envelope that was undeliverable, after they were opened so the original sender's info could be obtained. They're from the 1860's, and each in the set of six contains a city name: Augsburg, Bamberg, München, Nürnberg, Speyer, and Würzburg.

Earee's "Album Weeds" mentions these but he didn't think they were worth bothering with, since they weren't postage stamps. (What an unfortunate mindset!) He did mention that he had seen forgeries of these, and in a quick google image search I found a few that were just blatantly obvious.

However, I have seen some of these over the years that looked good but were marked "FAUX" on the back. I've seen some with just an "F" on the back - can someone confirm that means "faux" as well?

I am wondering if there is a reference someplace that outlines how to identify the forgeries of these stamps...? I actually have a few more of these, they look pretty much the same to my untrained eye, but the devil is in the details.

Thanks. Incidentally, I believe Michel lists these as RT1-6. I have the 2015/16 Germany Specialized catalogues, but they do not include the German states unfortunately...

Appreciate any help or guidance with these - I am not as familiar with German stamps as I would like.

Barry

"Faux" is french for "forged" or "forgery"
Michel catalogue lists two sets of those "Retourmarken" :
1-6 for those with the coat of arms (1865)
7-14 for those with text only,no illustration (1869/1905)

Michel # 10

Uknown authenticity.

Vayolene - I knew about "faux" (growing up in New Orleans, I've got some French vocabulary as well as genes ;) but I was wondering if the "F" alone indicates "FAUX" when just the "F" is on the back of a stamp? I don't have an example to scan and share, unfortunately, but I have seen this on ebay listings in the past. I did not know about the text-only retourmarken, thank you for that!

Rod - excellent!! Thank you! I thought I was so clever when I bought the English language version of the Michel Germany Specialized, but for some bizarre reason they omitted the German states from those and now I'll need the German language version to get those... but at least in the meantime I can refer to this for the catalogue info.

Amazing that someone would think it necessary to forge these. I have seen small groups (3-4 copies) of the Speyer label where you could tell "one of these things is not like the others", but then which is the genuine and which is the forged? I will keep digging. I really hate that Earee couldn't be bothered to give us a short verbal description of the forgeries, he spent more time and space in his book disparaging them than it would have just said "these are the forgeries" with a description...

Bcantin, the information Rod showed is from the Bayern Spezialkatalog, Band 2 by Peter Sem. This is not from Michel.