What were the S53P and S55P mic stands? When were they introduced?
Introduced in 1970 ($31.25), the S53P and the S55P were tripod stands designed for miking a stage.
The microphone was held in an air-filled rubber doughnut to isolate it from floor vibration. The mic was positioned parallel to the stage floor - about 1/8"" above the floor. This unusual positioning minimized comb filtering; it also increased the microphone output level because of the acoustic boundary effect provided by the stage floor.
For a complete explanation with diagrams, see the PDF file attached below.