Need help - Herman Miller shell with no stamp - age?  

hi - hoping someone on here can help me. i picked up 2 armshells at a garage sale the other day, both covered in black vinyl or leather. i was a bit disappointed when i got them home, however, because they do not bear any herman miller stamp on the bottom. because the vinyl/leather was splitting at all the seams, my husband removed the covers this morning, only to find a herman miller label attached to the inside of the upholstery. also, unlike my other upholstered shells, this one's foam and upholstery was attached to a fixed metal frame and not glued directly to the fiberglass. the bolt pattern on the bottom matches my other eames shells, though the shock mounts are larger in diameter. was there a period of production when herman miller didn't emboss the bottom of their fiberglass shells and mounted the upholstery to a metal frame, rather than with glue? if so, any idea the age of these shells? the date on the herman miller label is illegible ...
thanks in advance for any help!

p.s. also, the back of the shells is black, but the interior under the upholstery was a cream color, if that means anything ... thanks!