Somebody above said the first KR was 1928. That's not really correct. The 1928 opposed 14 cu in was called an A but the second digit R didn't appear until 1931. That was the KR55. It was and alternate firing 14 Cu in, internal rotary valve. The KR55 was really the only KR ever built. It was a 1931. The subsequent engies were just selling off inventory or built as required.

The numbering system was:
KR55 =1931,
KR60 = 1932, etc. until KR80 = 1938.

It is noteworthy that th K series were only 14 cu in. It wasn't until about '51 the we raised the allowable size to 15 ci, to "let" those Mercury guys with their KG4s run with us. In 1961 Jack Leek set a 61 MPH record with a modified Mercury KG4.

Pep Hubbell, and maybe some other people made the 15 inch blocks. The KR55s had a 2.125 bore the 15 inchers had a 2.44.