Evan,
What information do you need? I can give you historical information, but Tom, Fred, or John could help you more than I could if you need repair info.
For starters, the 300 was made from 1935 to 1938. There were 274,788 of them made, and they were the economy model selling next to the midrange 475 and top of the line 825, and later, the mid range 25 and top end 150. The 25 as mentioned, was the midrange model made in 1937 and 38. 114,354 of them were made.
The 26 was made from 1938 to 1941, and was the midrange model in between the 305 (economy) and 60 (top of the line). There were 253,768 made, and looks like yours has a nice original bag.
The 27 was Hoover's re-entry into vacuum manufacturing after WWII. There were 179,229 made in 1945 and 1946.
And finally, the 28, was one of Hoover's most popular models and one of my favorites, and one that all collectors should have. There were 2,278,595 made from 1946 to 1950. It was the economy model selling next to the model 61, and became the midrange model after the 115 was introduced in 1948.
Hope this is helpful.
Jeff