Matchbox Superfast 66d2s Greyhound Coach Grey Sticker Set.
The Greyhound bus or coach #66 was issued with both Regular Wheels and later with Superfast Wheels.
The stickers or decals are printed as either a blue logo and stripes or in a mid-grey colour.
They are commonly applied as decals for regular wheels and stickers for the Superfast models. However this rule is not consistent and both stickers and decals have been noted on both Regulare and Superfast wheel models. This would appear to be a case of whatever the printer had available was used.
RecoverToy offers all 4 options:
Decals 66c1d in Blue
Decals 66c2d in Grey
Stickers 66d1s in Blue
Stickers 66d2s in Grey
This is so you can make a choice of colour and format for your model.