John Hermanson wrote:

My repair database shows OM-1 chrome ending around 1,593,xxx. The 1N
starting around 1,644,xxx. The start of 1N production also signaled the
start of the factory using prism foam again.

OM-1 were made with no prism foam from (approx) 1,119,xxx (3/77) through
1,623,xxx (11/78).
FWIW I had the prism changed on my OM1n due to foam rot when Mke Spencer CLAd it.