Saving Old Seagulls

I recently found a CPC powerhead assembly (CPCL1471E4) which had attached a cylinder with visibly larger transfer ports than those found on my own CPC (CPC136A6), and also a larger, bridged exhaust port. The cylinder head was different, too, in that it had a raised internal area to fit the contour of the piston, presumably for a higher compression ratio. Were these parts a later redesign to produce more power, or perhaps they composed the "plus" in Century Plus?

Todd