The last letter of their mark series' are what Auran Kultasepät called 'kretsileima'. The closest translation of this is 'filings mark', specifically the waste filings or rubbish left after making a metal object. They were originally associated with being a mark for identifying the responsibility for shoddy and bad work rather than good; an internal quality or control mark if you like. Only later were these given 'master'-mark status.
n = Veikko Nuutinen (1941-1954)
x = Matti Lehto (n.1950-1989)
e = Raimo Roine (1946-1990)
m = Risto Nordling
A = Armas Ahla
A = Anna-Mari Autio (1980s)
B = Aimo Järvinen
C = Reino Lehtinen
E = Aulis Alin
K = Antero Kivi
L = Eero Lehtonen
P = Kauko Stranden (1950s)
P = Pertti Alin (1975-1990s)
R = Antero Rimmanen
U = Sulevi Ojala
Ä = Matti Salonen
Å = Harri Ahokas
UNKNOWN
M, F, J, G, O, H