The oval decals were introduced for the titanium models when they hit the market in 2005 and limited to them. Steel Bromptons had rectangular ones from the beginning until today. Both types of deacl have changed in design somewhat over time, this could theoretically be used as a rough indicator for the age of the bike. In practice not so much as you'd have to see both sides of the frame and decals can be bought as a spare, so the decal design may not correlate with the bike's age. Furthermore sometimes ti Bromptons got rectangular decals from factory for whatever reason (I own one of those, bought new). Steel Bromptons with an oval decal are either former ti-Bromptons that got robbed of their ti-parts or the decal was bought aftermarket.