Among the topics that pique my Vacuumland interest are posters' personal recollections of cleaners. So this thread has been a fun read.
Our city only had an Electrolux Branch Office for a short time in the '60s till maybe the early '70s according to the lists in the Model F, Model R, Model G, & Model L user manuals (1957 to 1967), so they weren't that popular as others like Kirby and Kenmore, but I had a passing interest in cleaners as a youth and only saw my first 1205 in person several years ago.
Anyway, Electrolux had the practice of updating user manuals as changes and upgrades were made. A couple 1205 manuals were printed in 1968, and one annually each year through 1974. I have most, so using them, personal observations, and knowledge from other Vacuumland posters, here's an approximate timeline of notable changes & upgrades, although there may have been some transitional iterations:
1 - 1968: smooth tank/cylinder body, Model G motor, red printing and border on exhaust cover, serial number on bottom of back cover trim, black Automatic Control interface, turquoise bag door w/pn port under hose inlet, standard coupler hose with metal goose neck grip with/without banded pn cord, PN-1: teal blue cover w/white plastic wand
2 - 1968-1969: serial number on stabilizer, power hose & coupler with plastic pistol grip, power hose with standard coupler & pigtail plug, or standard hose & coupler with plastic pistol grip; PN-1: w/ribbed teal blue elbow, telescopic wand & teal blue ribbed sheath
3 - 1969-1971: 1205 motor, red printing on exhaust cover (no border), jade Automatic Control interface, jade bag door, PN-1 w/smooth teal blue elbow & wand sheath, & steel wand
4 - 1972-1973: black exhaust cover w/no red, power hose & coupler with metal goose neck grip and plastic pn port or standard hose with metal goose neck grip
5 - 1973-1974: textured tank/cylinder body, PN-2 w/smooth jade elbow & teal blue wand sheath & cord; & steel wand
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All iterations had the same colored hose: teal blue with 2 white tracers; though the last runs may have been accessorized with the white hose paired with the 50th Jubilee 1205 tank cleaner.
Optional accessories throughout the run were the Rug Washer single brush dry foam shampooer, sprayer, vaporizer, & the white-bristled dusting and wall brushes. The separate triple brush Model B-8 dry foam Carpet Beautifier-Polisher was also available.
Clarification: a 'tank' (or 'cylinder') cleaner's motor is mounted horizontally; a 'canister' cleaner's is mounted vertically.
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Unfortunately, Electrolux Corporation chose not to publish a timeline of each 2-letter series comprised in the serial numbers; although I suspect at least some may be listed in service manuals. Such a roster could also be composed using the customer purchase histories kept by the company. It seems to me that the 1205s made in the '60s had serial numbers with 'Y' prefix, and those manufactured in the '70s had serial numbers with the 'Z' prefix.