Oreston / Pentacon 50/1.8 was first manufactured for the Exakta RTL-1000 in auto mount, and later in M42, M42 Electric, and later (until the late 80's) in Praktica B mount.
This lens has several interesting features, it can focus right down to 33 cm , good enough for most close up; lesser known is the fact that the lens has almost perfect symmetrical design very similar to the original Zeiss double Gauss Planar design (with thick cemented center groups) making it a very usable lens for reverse mount and bellows work. In fact it was remarked by many experienced in this lens that it performs excellently in the macro / close up range.