G-E USA & Canada introduced their new "Airflo" tank-type cleaners in late 1939 according to newspaper ads and consumer reports from an electronic database. I'm unsure if both countries' models & IDs were identical, but the model ID and motor ratings of the US's was the AVT150 and 500-watts.
The earliest mention of your 600-watt model AVT701 "Airflo" in the database was a 1942 OFFICE OF PRICE ADMINISTRATION publication; albeit the US government literature was not completely scanned, so the "snippet views" provided instead may have omitted that ID and obscured a possible 1939 introduction.
It seems that both models were succeeded in 1948 with the new models AVT172 "Airflo" and 14-inch long AVT160 "Tidy" with the throw-away and "telescope" dirt bags, respectively. The tank-type line was likely dropped around the time new Swivel-Top canister model(s) was (were) launched in 1950 (CA) and 1951 (US).
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Anyway, '6134' is the serial number and likely indicates order of production. Its price value is whatever someone is willing to pay for it like any other used household item. As the saying goes, "Beauty is in the eye of the beholder." Your grandparents' G-E is in apparent fair condition and the original attachment set does add value (the original manual and paper work would add even more).
Photos:
1. 1939 Nov 25 DAYTONA BEACH MORNING JOURNAL - ad (AVT150?)
2. 1942 OFFICE OF PRICE ADMINISTRATION document - Eureka (partial), Filter Queen, Filtex, and G-E vacuum cleaner models price lists
3. 1947 Oct 13 LIFE ad (AVT701?)
