Baader's # T2-23 # 1508015 adapter converts the external M42x1 threads of many Russian telescopes into an external M42x.75 T-2 thread. The M42 x 1 thread is also common to Pentax threaded camera lenses.
Specifications:
Total adapter length = 12mm (0.47")
Optical length = 7.5mm
Thickness of knurled ring = 6.5mm
Length of male T2 thread = 4.5mm
Length of female M42x1 thread = approx. 4.5mm 
Maximum outside diameter = 54.7mm (2.15")
Weight = 0.7oz.