Swivel caster is indeed standard On Diamond Jubilee
The swivel caster is standard on the diamond jubilee. It should be surrounded by two glides that are plastic. The much older model G has a swivel wheel in front but also two wheels that were there for stability to prevent tipping and to glide the vac on thicker carpet. They are fully operational wheels in addition to the swivel caster.
When the model 1205 was introduced in 1968 it’s design turned the Bag horizontally so as to help with the tipping over problem from the previous design and the plastic glides with the swivel wheel in the center was standard on it exactly as you see it on your Diamond Jubilee. The tipping problem over the early models had been something the engineers battled with since the introduction of the metal body design with the model E which had only the swivel caster for a front wheel. Then they added some flat glides to the E and AE flanking the swivel wheel. They changed the glides to angled metal ones for the Next model, the F still surrounded by the swivel wheel and then they changed those metal glides to two regular wheels aside the swivel wheel on the G as the F’s metal glides could scrape flooring but were a definite improvement from the E and AE design.
On lower level models the S has only a single forward wheel in closed in a metal skirt. The R changed this to two small front wheels but close together and no swivel, then wider double wheels for the L. These models all had improved double front wheels for even more stability but none of these swiveled so the tipping problem was still an issue especially so on these lower level models. They put the swivel casters only on the higher, more expensive models as a selling point until the the lower level models finally got a redesign as the Brown model 1453 came out changing the bag design to horizontal in 1980 and having the same plastic glides with swivel wheel in the center on it that was standard on its upper level twin, the Olympia One. Pretty much since then, every model has the front swivel wheel surrounded by two glides to avoid any sort of tipping.
If you see a Horizontal bag model with the front glides but no swivel wheel between them, then the swivel wheel has been removed by someone or otherwise damaged.
The last model with no swivel wheel was made in 1979 and was the Gold version of the model L with 4 wheels and the front ones were wide but smaller diameter and mounted close to each other.
Earliest models had metal rails and did not include wheels, but some were added as options either made by Electrolux for some models or by third party companies to fit Electrolux as some people did not like the idea of dragging the machine behind them on their finished hardwoods on metal rails fearing the rails would damage the floors so eventually everything went full wheels.
I do find it interesting that the model E which was the lower level model, had the swivel wheel, but the later low level models it was eliminated. The AE was the Automatic control version of the E and also had the swivel wheel. The thrift model T only had a single glide in the front. It seems Electrolux stripped down the lower level models after the E for some reason, likely because once they came out with the E it started the two tiers of models and perhaps they wanted more than just the Automatic control feature to be a feature of the higher model to sell more higher level models otherwise the two models would be too similar to justify a higher price and entice buyers to it. Note also the cord halo and cordwinder option were not available or equipped on the Thrift model or model S...another “cheapening” of these lower models. My parents have a model S, it is incredibly light with its simple design, no filter, no cord halo, only the single straight front wheel. Even the bag door is a simple clip on light metal.
Jon