I am not 100% sure but I think 38 m/m (1 1/2" ) is the board thickness, prior to having the laminates attached.
I always buy the wider worktop 665 m/m to allow more room behind the units for pipes/electrics.
And to provide a slight overhang of the units at the front, which I think looks better.
There are all sorts of thicknesses available from different manufacturers, from a relatively cheap and cheerful 28mm thickness upwards.
The thickness can also vary across the width of a worktop, as the laminate may continue a few inches round a bullnose 'front' profile where it's on show, but not continue across the full width of the underside.
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