My WAG here is that the "D" is correct and likely indicates that the horn was aquaplas coated.
The system is rated at 50 watts continuous with an 808 driver, at 50 watts in the 500 hz range, the 808 probably produces enough acoustic energy that it audibly could "ring" the horn. For how long it will continue to do it at 50 watts input is another debate.
After enough warranty claims for blown 808's, they sawed off the small format throat and flange and created the 511E such that the system could run the 291 driver same as the A-10 system.
This of course is all just my wild assed guess...........................
FWIW, you will also find an 811C and 511C mentioned on the sales slick for some of the portable SR speakers.
http://www.greatplainsaudio.com/vint...spec_sheet.pdf
That suggests to me that the suffix letter had nothing to do with paint color or mounting holes.
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