A bit late, but my brief history of New Beats:
http://black.net.nz/avedis/new-beats.html#NBtop
If you want to date later ones than those covered in the New Beats page, then follow the links (at the bottom) back to my gallery or timeline by years to check out ink and laser serial years.
I've got lots of additional weight info which shows that yes, they got heavier over the decades.