I used to have two Model 28s. They were slightly different:
==VERSION ONE==
Serial number 2967316
Serial number plate silver color w/ dark brown lettering; wording different from mottled version
Motor bumper very dark gray rubber w/ ribbed top surface
Hoover logo under headlight housing gold color; background color of metal dark brown
"Fan" shaped area under headlight dark brown smooth bakelite
Painted base brown "wrinkle finish"
Motor hood very smooth brown bakelite; alternating gold and dark-brown stripes actually painted or decal'ed on the bakelite instead of a separate metal strip
Handle grip light brown plastic
Both agitator brush strips straight; beater bars curved
Bag connector-ring made of metal - painted dark brown
Brown cord with Hoover logo on plug
Brown cloth bag
==VERSION TWO==
Serial number 3957014
Serial number plate gold color w/ dark brown lettering; wording different from smooth version
Motor bumper medium brown rubber w/ smooth top surface
Hoover logo under headlight housing platinum color; background color of metal dark brown
"Fan" shaped area under headlight dark brown mottled bakelite
Painted base brown "wrinkle finish"
Motor hood very mottled brown; silver stripes on separate dark brown metal strip riveted around bottom edge
Handle grip dark brown plastic
Both agitator brush strips curved; beater bars curved
Bag connector-ring made of mottled brown bakelite
Brown cord with Hoover logo on plug
Brown cloth bag
Here's a video of collector Roger Proehl showing his beautiful Model 28. (Note: It's a 60-mb video so it will take a couple of minutes to load.)
http://www.1377731.com/0000137/roger.hoover28.avi