One minor correction to one of Tim's points. The short side cover actually began with the 235 redesign in 1950. The '50-'53 235's had the short side cover, but were still low pressure (excl. '53 PG).
Todd: The I9 in your serial number is just part of the sequence number. Many times the letter I was used instead of the number 1. That means your full sequence number is 426919, which means it was the 425919th engine built at Tonawanda for the '53 model year (the numbering started at 1001).
