The taps labeled "Sc" are likely intended to be connected to the screen grids of the output tubes, to achieve what is commonly known as "ultra-linear" operation. This is a form of distributed loading originally patented by Alan Blumenthal, but then later developed into a product by Hafler and Keroes:
https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_02/073_Keroes_Hafler_UL_US_1955.pdfThe screen tap connection provides a form of feedback that reduces distortion and also reduces output impedance of a pentode amplifier. It slightly reduces the efficiency, below that of a pentode, but provides much more linearity and lower distortion than the triode connection, at almost the power level of a pure pentode stage. It was used for many years and was a common upgrade to the D T N Williamson amplifier circuit. One caveat is that the tubes chosen must have a screen voltage rating almost as high as the plate voltage rating, if maximum output is to be realized. Acrosound, one of Hafler's and Keroes' companies, (Along with Dynaco) produced a multitude of high quality output transformers, as well as amplifiers employing this design feature. For 6146s, they produced the TO-350 transformer, which had a separate screen winding, thus allowing the screen grids to be powered from a separate, lower voltage, regulated source.
You will find many circuits applicable to your transformer here:
http://www.tubebooks.org/file_downloads/acrosound.pdfThe example with push-pull 807s using the TO-300 transformer was very effective when driving a quad of Taylor TZ-40 class-B modulators in push-pull parallel, rivaling the original speech amplifier employing a quad of 2A3s.
