French Merson-Type Stamps Of 1916 Montenegro Government-In-Exile

In 1916 Montenegro formed a government-in-exile in Bordeaux France. Maury lists three French Merson-type stamps – identified as 9, 10, 11 – associated with the exiled government. These are overprinted, "S. P. / du M. / Bordeaux". I'm guessing that "du M." is "du Montenegro", but the "S. P."? Could the initials be "Service des Postes" or "Service des Postes en France"? Any guidance would be appreciated.

"Service Postal du Montenegro"
Happy New Year.

Welcome XYZYX ......Can we get a scan of the stamp , The only S.P. overprint that I know was done for Official Stamps . That would stand for Service Public .

These overprints were never officially recognized by the French administration and were susequently outlawed.They would be considered as
a somewhat fantasy type of creation ,although some (Rare) were found used in official mail in 1916.They exist in 7 Sower and 4 Merson types .
'Service Postal du Monténégro - Bordeaux'

They were Official stamps from June 6th to the 27th 1916 ,there was 11 stamps to the complete set values from 5c. to 50c.

There also was a return letter label/stamps one in black on blue paper and one Black on red paper .

Thank you - floortrader, perf12 and vayolene - for your informations. Although I failed the "S.P." quest on my own, I did develop a much greater appreciation of the complexities of World War I in the process.

Here are two return labels (fantasy,just like the stamps)