One production 1-23H was rebuilt as an experimental aircraft in the "racing-exhibition" category by Sterling Starr and designated SGS 1-23HM.
The HM was built by using a stock 1-23H fuselage and tail assembly. Starr built a new set of 54 foot (16.6 m) span wings with an aspect ratio of 23.5:1. The wings took 1500 hours to construct, but the resulting aircraft has a glide ratio of 33.2:1 which is a 7% improvement over the stock 1-23H. The HM later had retractable landing gear added.
The 1-23HM was flown in the 1966 US nationals.
The aircraft is registered as N94298 and is currently on exhibit at the National Soaring Museum, Elmira, NY