Polskie Nagrania Muza

Polish state-owned label in the People's Republic times, still active these days (releasing mostly the old, remastered recordings, classical music, operas, etc.).
Currently the brand is owned by Polish branch of Warner Music.

Some useful information which could be helpful when establishing the year of release of LPs (up to year 1991) or at least the years range:
Prefix
SP - 7'' / 45 rpm mono single play
SS - same line as above, but in stereo (from mid 70's)
N - 7'' / 45 rpm mono EP
SN - same line as above, but in stereo (from mid 70's)

L - used for 10” / 33rpm longplays

for 12'' longplays:
XL - used for mono LPs – up to 1975/76
SXL - used for stereo LPs – up to 1975/76
X - used for mono LPs (mainly reissues) – after 1975/1976
SX - used for stereo LPs (reissues and new releases) – after 1975/1976
In the transition period quite often there was a different prefix printed on the sleeve ( XL/SXL or SXL) and on the centre label (X or SX).

Matrix numbers
For early mono releases prefix XW was used for matrix number (printed on labels and etched on runouts).
When stereo records were introduced, there was M-3 XW for mono and S-3XW for stereo records printed on labels and respectively M3