Based on when the Hoover Concept One U3101 model appeared in advertisements from newspapers and sales flyers that I can locate, which was from 12 July 1978 to 4 Nov 1984, and when the instruction manual was printed (1978), and based on
@paul 's description of the post-1967 serial number formatting ("one for the last digit of the year, and two for the month"), and
@dirtmaster37 saying they used production numbers of 5,000, I can go along and pick that out of the serial number and try to make sense of it.
Without knowing what color yours is, I can not find anything of a Hoover U3101/Concept One existing in 1977 in any correspondence regardless of color, so it absolutely was not on the market until summer 1978 at the very least. It for sure would have showed up in Christmas sales rush advertisements if it was ready to roll out in Dec 1977, so there being none for the holiday sales season means it did not exist for purchase yet (assuming there is no more earlier correspondence to be found that has fallen into the digital void). Likewise this model was never mentioned again after summer 1984. So I can presume this U3101 was sold from 1978 through 1984 whether at retail or in discount stores after it aged out. This makes sense because the Concept II came out in 1983.
I found two more U3101'S on eBay - all blue with the floral motif on the bag door.
00540373 - blue w/motif
00258333 - blue w/motif
Based on advertisements, we know the the blue one with the motif came out in 1983.
From this I think I can work out the date as such, where 'u' is unit number, 'ww' is week and 'y' is year;
uuuuu/ww/y
---------------
00540/37/3
00258/33/3
This matches the 1983 production and sales of that specific vacuum.
So plugging in this now-known formatting into your vacuum in specific;
00020277
becomes
00020/27/7
The best I can come up with would be 27th week of 1977, which would be July 4, 1977 to July 10, 1977. With a vacuum production number of the 20th one that rolled off the line that week. 20 vacuums divided by 7 days would be 2.9 vacuums per day - lets just say about 3 - thats just for the Concept One product line of course. That seems reasonable. Also just FYI, "Star Wars" came out 2 months before this Hoover, so we know what those employees were doing!
It does appear from what I am reading that 1977 was also the same year that then-CEO Merle Rawson moved Hoover Worldwide from New York back to North Canton, OH, which might account for delays in shipments to where even though the vacuum rolled off the assembly line in Summer 1977, they were stockpiled and didnt ship out until spring of the following year. Hoover had experienced a oversaturation of the appliance market both from themselves and other makers in 1974 and 1975 which resulted in stagnant sales for the company and a stock slump for 1976/1977, which is why I guess he made the decision to relocate the company HQ in 1977. This of course would delay any and all shipments because return addresses and company mailings would all need to be re-corrected and re-labeled with the new address.
This is just an idea I came up with on how the serial numbers are formatted (on the Concept One at least) from an hour and a half of research so I'm not going to make any claims that it's God's bible, but it does make sense insofar. Maybe it helps? Maybe not. Good luck!