Apparently the expression "Plumbers Crack" is now officially in the Oxford English Dictionary.
Other expressions noted are "plumber's cleavage" (and "brickie's cleavage"), "refrigerator repairman's bottom", and "electrician's butt". In Australia it's the "coin slot", or "builder's smile". In Dutch its "bouwvakkersdecolleté", which translates into English as "builder's cleavage"
So now you know.................