I have never heard baseball caps called that but there is a line of baseball caps marketed by New Era that are called '59FIFTY'.
Baseball...
mark has 61 melinda has 54
Melinda has 54, Mark has 61.
CAPS-R-US
Baseball caps have quite a large market. You can purchase baseball caps from buckwholesale.com. I would also suggest looking at alienexpress.com for wholesale baseball caps as well.
Men want to buy baseball caps with team logos on them.
Straw
Straw
they still do. and to keep the sun out of their eyes
they don't
As an easy way for players/clubhouse staff to recognize which is which....