The small black seal affixed to the screw hole at the 12 o'clock position was designed to be a tamper-evident seal. It appears on all but the earliest M3s until the M4-2. A raised 'L' signifies a factory seal. An indented 'L' means that it had been back to Leitz for repair or service.
An intact seal, especially a factory seal can be a good or bad thing, depending on how you look at it. A raised seal means that the camera has never been opened and never subject to sub-standard repair work. It is also proof that the camera is long overdue a service!
It is theoretically possible to save the seal, perhaps by trying remove the seal intact, to be replaced later. We have never tried this and I doubt this is possible. We've heard stories about the possibility of servicing the camera without removing the seal. Since the screw under the seal goes through a tab connected to the top plate, then either the top plate stays on or the tab is broken, so that the seal does not need to be touched.
If the seal offers some sort of perceived value, because you know the camera has never been tampered with, I would suggest that you take a picture of the camera with the original seal in place, to be used as provenance, should you decide to sell the camera later. Attempting to replicate the original after a service would be dishonest. We only use our own 'W' seal. We do see cameras with a plain seal, which is better than leaving the recessed screw head exposed and is quite common.
The practice of fitting seals ended in the early 1980's. We believe cameraworks-uk are the only repair shop to still use a seal. It is used for its original purpose. It is a tamper-evident seal and is an indented sans serif 'W'.
Below are some well-known seals. Many of these are long out of business. There are some duplications, some letters have been used by more than one organisation. We believe the letters were designated by Leitz. Earning one's special seal would have been cause for celebration back in the day. It is one tradition that we intend to keep alive.
69: As yet, undetermined
92: (Raised 92) Don Goldberg DAG
A: Alois Kosnar, Stockholm
A: Venzlaff, Athens, Greece
B: Leitz (Leica Mayfair), London UK
C: Manufactured by Leitz (Leica) Midland, Canada
C: Leitz, Milan, Italy
D: Tiranty, Paris, France
E: Brandt, Stockholm, Sweden
F: Lutz Ferrando, Buenos Aires
G: SAMCA Photo Services, Genoa, Italy. Thanks to Alex Roggero/Gianni Medici
H: Havel Camera, Texas, USA (closed down)
H: Schmidt, Hong Kong
I: Taeuber, Cape Town, South Africa
J: (Raised J), Gerry Smith, Toronto (sadly, passed away in 2022).
K: Kanto Camera Service, Japan
K: Kleiner, Rio de Janeiro, Brazil
L: Raised letter, manufactured by Leitz up to M4-2
L: Recessed letter, Leitz service dept up to 1982
M: (Indented sans serif) Leica Manchester UK
M: Pablo Ferrando, Montevideo, Uruguay
N: Leitz New York
N: Lackland, Aukland, New Zealand
O: Odin, Nijmegen, Netherlands
O: Unknown, Adelaide, Australia
P: Perrot, Biel/Bienne, Switzerland
Q: Parer, Sydney, Australia
R: Sanitas, Bogota, Columbia
S: Hand-engraved letter, Sherry Krauter
S: Schmidt, Tokyo, Japan
T: Carveth, Toronto/Canada
U: Commercial Ultramar, Mexico
V: Forestier, Valparaiso, Chile
W: (indented sans serif) cameraworks-uk, UK
W: Optoteknika, Vienna, Austria
X: Fixit, Bagdad, Iraq
Y: Leitz, New York
Z: Reiss & Co., Jakarta, Malaysia