From Al Cosentino's "Fiat SOHC Race World" (typed by Courtney):

"Fiat found combustion chamber shape is critical for engine
response.  The first European 1290 cc 128 heads had a flat area
opposite the spark plug.  All USA versions have the combustion
chamber shape shown - the recessed area in the head (right side
of drawing) brought three more DIN horsepower with the same
compression ratio (to 73 DIN or 83 SAE with 1290 cc engine).
This doesn't compare with the power gained by using the early
1116 cc head to gain compression.  The early head doesn't have
the recessed area which is milled out of the casting for current
engines and fits all the SOHC 1290 and 1498 cc engines.
Although the early head doesn't have as efficient a combustion
chamber, you more than make up for it with the increased
compression ratio."