Since undertaking this project, one of my big questions has been why the RSTL and RSTLM are designed to be angled such that they cross in front of the listening position. I finally found the answer in the 1989 Stereophile review of the RSTL:
When properly positioned, the front baffle, with its mid- and high-frequency drivers, is 45° off-axis from the listener. This scheme follows Blumlein’s recommendations for stereo reproduction. To increase dispersion, the supertweeter augments high-frequency output off-axis. Consequently, when measured on-0axis, the RS has has a rise in high-frequency response which the manufacturer says disappears at the listening position.
Interesting. I’d imagine this would be stunning when playing back a recording made with a Blumlein Pair, but it is not obvious to me that it would have technical advantages otherwise.