You  are  right  about  the 'magnum blend'   not  as  bad from  environ metal but some  nitros that I  have  looked at  look like  floor  sweepings blend. 
here is  a  picture of the  choke tube... and  it  might  be  seminole  choke ,  I  always thought it  was  briley but  the  stamping  look  exactly  like  the  seminole writing. 
the  constriction was made  by using  a  caliper  that measures  exactly like  the   stamped 675 
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