There were two Route 15A's, one in Portland and one in Union.

The Portland Route 15A was created about 1938, when Route 17 was called Route 15. Route 15A was the section of 17 between the 17/17A junction and Route 66. (Curiously enough, Route 17A was part of plain Route 15.) By 1940, Route 15 and 15A were switched in Portland, and 15A ran where 17A does now. On May 1, 1948, Route 17 was created from the old Route 15 from Glastonbury to New Haven, and 15A became 17A. (Have a headache yet? :)

The Union Route 15A arrived in 1940 as the 2-lane Wilbur Cross Highway started construction near the state line. In 1948, it became part of Route 198, and in 1963 part of Route 171 (north of Route 190).