10-Inch "Mannheim-Trig" Slide Rule
Frederick Post model 1457[1945]
Same as Roos SR-109

On Roos-made rules the red ink fades easily and the celluloid facing turns very yellow. 


Excerpt from 1945(?) Price List 
Second price is per dozen

Note that this rule isn't "duplex" in the usual sense.  There are no scales on the backs of the stators, and for many operations, one would reverse the sldie just as if it were an ordinarly closed-body rule.