<s>
The	O
M39	B-Algorithm
lens	I-Algorithm
mount	I-Algorithm
is	O
a	O
screw	O
thread	O
mounting	O
system	O
for	O
attaching	O
lenses	O
to	O
35	O
mm	O
cameras	O
,	O
primarily	O
rangefinder	B-Algorithm
(	O
RF	O
)	O
Leicas	O
.	O
</s>
<s>
True	O
Leica	B-Algorithm
Thread-Mount	I-Algorithm
(	O
LTM	O
)	O
is	O
39mm	O
in	O
diameter	O
and	O
has	O
a	O
thread	O
of	O
26	O
turns-per-inch	O
or	O
threads-per-inch	O
(	O
tpi	O
)	O
(	O
approximately	O
0.977mm	O
pitch	O
)	O
of	O
Whitworth	O
thread	O
form	O
.	O
</s>
<s>
The	O
Soviets	O
later	O
adopted	O
the	O
LTM	O
mount	O
for	O
their	O
Zenit	B-Application
single-lens-reflex	O
(	O
SLR	O
)	O
cameras	O
,	O
though	O
with	O
the	O
longer	O
optical	O
registration	O
of	O
45.2mm	O
,	O
required	O
to	O
allow	O
the	O
mirror	O
room	O
to	O
flip	O
out	O
of	O
the	O
focal	O
path	O
when	O
a	O
picture	O
was	O
taken	O
.	O
</s>
<s>
The	O
mount	O
was	O
developed	O
by	O
Oskar	O
Barnack	O
at	O
Leica	O
to	O
provide	O
a	O
system	O
that	O
would	O
allow	O
for	O
the	O
exchange	O
of	O
lenses	O
on	O
their	O
new	O
small	O
film	O
cameras	O
(	O
Leica	O
Type	O
1	O
and	O
Leica	O
Type	O
2	O
)	O
,	O
as	O
Zeiss	O
Ikon	O
had	O
indicated	O
that	O
their	O
forthcoming	O
Contax	O
rangefinder	B-Algorithm
cameras	I-Algorithm
would	O
have	O
interchangeable	O
lenses	O
.	O
</s>
<s>
The	O
test	O
marketing	O
program	O
was	O
a	O
success	O
,	O
so	O
regular	O
production	O
with	O
Leitz	O
camera	O
bodies	O
and	O
lenses	O
was	O
introduced	O
on	O
the	O
Leica	B-Algorithm
II	I-Algorithm
,	O
and	O
featured	O
on	O
the	O
Leica	B-Algorithm
Standard	I-Algorithm
and	O
Leica	B-Algorithm
III	I-Algorithm
.	O
</s>
<s>
Until	O
the	O
1970s	O
the	O
39mm	O
mount	O
was	O
the	O
norm	O
for	O
exchangeable	O
lenses	O
in	O
rangefinder	B-Algorithm
cameras	I-Algorithm
.	O
</s>
<s>
Currently	O
,	O
Cosina	O
in	O
Japan	O
—	O
many	O
of	O
whose	O
recent	O
products	O
are	O
marketed	O
under	O
the	O
Voigtländer	O
brand	O
—	O
and	O
FED	O
in	O
Ukraine	O
continue	O
to	O
produce	O
camera	O
bodies	O
using	O
a	O
true	O
Leica	B-Algorithm
thread-mount	I-Algorithm
.	O
</s>
