The Summarit was originally Leica's 5cm F1.5 lens based on the 5cm F1.5 Xenon, which was built under license to Taylor, Taylor and Hobbs. I've seen it stated that it was "just a coated Xenon" and I've read in Morgan and Lester that it was an improvement over the original Xenon. It is a fine performer, and has a "interesting Different" look. The 5cm F1.5 Xenon and Summarit are a 7-element in 5 group lenses that took the original 6/4 Xenon rear most element and split it into two lenses each of lower power (source: Neblette, "Photographic Lenses"). The "classic" Summarit evolved into the Summilux.
Leica resurrected the Summarit name for the Minilux a few years ago, but it was an F2.4 lens and was back to the classic 6/4 configuration. It performed like a Summarit, just not as fast.