Yep, PGI's interlocking reverse-L (Greek key) mold -- and the materials they used at the time -- rank among the best china clays ever produced.
But -- the chips in the OP are NOT the PGI china clays...... those are ProGen 80 chips, a cheaper chip produced on the interlocking-J mold using inferior materials. Still a decent buy at 10c each, but not the great bargain that PGI chips would be at that price.
@MegaTon44 should probably correct the thread title.
ProGen 80
PGI greek key (4D14-spotted version)
PGI greek key (8V-spotted version)