Academy Tiles are always my first point of call for tiles that are not stocked by the generic tile retailers. The domed finish you speak of is by the build-up of glaze on a flat circular porcelain tile. The glaze towards the edge of the tile is thinner than the glaze towards the middle which gives a subtle "domed" appearance.
Although I wouldn't be using penny round tiles in a 1969 retro house if I was trying to be authentic as they are too contemporary.
Dr Retro House Calls
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