[pure-silver] Mamiya Macro

I purchased a second hand 80mm 1:4 Mamiya Macro lens for my 645.   Which I 
intend to use both with and without extension tubes.
 
I am looking for information on this lens.
 
I did locate one instruction manual on the web but it did not  address an 
extra feature on this lens which I find most curious.
 
On the front of the lens barrel is a ring labeled MAG  SPACER which has an N 
and numbers 1, 1.1,  1.25, 1.5, 1.7, and 2.  It also has a release button 
which seems  to only work when the lens has been extended (close focus).  There 
 
is a third set of values next to the focal distant values labeled  MAG which 
range in value from 2 to 40.
 
Could some one point me to more information on the usage of this  lens?
 
Thank you 
Alan Tippett, San Jose



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