1986 Miyata catalog shows it coming (in that color) in 46cm and 50cm, nothing in between...but of course the method of measuring varies. For that matter in some years the Miyata catalogs show metric measurements, sometimes it's inches. www.miyatacatalogs.com

In 1987 Miyata went to the 'Terry' style of 24" front wheels for small road frames.

CVer Bibliobob had a Shogun 400 with a similar small frame up for sale in Chicago during the summer, and I bet a search would bring up its picture for a comparison.

Nice looking bike.