You were asking about photos; here's some of another GU-12, from an eBay auction:
http://cgi.ebay.com/Vintage-Ampeg-Tube-Amp-GU12-Reverb-Very-Good-shape_W0QQitemZ7379879926QQcategoryZ43372QQrdZ1QQcmdZViewItem
My GU-12 has the original knobs (the usual round black pointer knobs with the brushed insert), and mine has never sat in a flooded basement, as appears to be the case with this one... :(
edit: "Should be putting out somewhere around 20 watts."? Rated plate dissipation on a 7591 is 19 watts; I think he guessed a little low...
C.K.