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I can strongly recommend this fine lens (even though this one has 14 blades as stated by the seller and mine just has 12?).
Very interesting manual, preset lens to try out and right now at ca. 6US in this auction:
RARE *14 BLADES* CARL ZEISS TESSAR 2.8/50 *M42* | eBay
Samples etc here:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/120768-carl...-old-lens.html 
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