Willie,
Not sure I can add more than you know. I also own the very same manifold; I've never seen a 49-53 version of it (but that doesn't mean they don't exist).Obviously a street design for generator clearance. The one I have has the passages immediately below the carb flanges bored out to link the upper and lower plenums together. Since the rear carb is positioned over runners 3/4 &7/8 and the front carb is positioned further back fuel distribution may be biased to the rear. I've never run mine to determine the a/f distribution. I'm not sure if the boring out of the plenum floors was a factory modification or not. Albert Drake did a small book years ago that was a compilation of old speed shop and equipment adds but that manifold was not listed. I suspect that it originated as a standard 180 degree design and was copied(and perhaps modified) by Power engineering. That seemed to be pretty common practice in that era. There are no date codes on mine to shed any light on manufacturing dates. Just guessing I'd lean to post war, pre 1950 era.......Iron Mike