Back in the day, when more cream in milk was considered a plus and before homogenization, milk bottles were designed to show off the cream that rose to the top. Thus the early bottles had tapered necks to lengthen the cream line.  Then the Cream Top bottle was designed and patented with a bulb on the top to enhance the appearance of the cream as it rose to the top. Soon after, this little metal spoon was designed and patented. It was designed to both ladle off the cream by the spoonful but also when it was carefully inserted into the bulb it's round base would seal off the bulb so that the cream could be poured off without mixing it with the milk below.  

This "Cream Top" concept was controlled by the patent holder and licensed to a very few bottle makers who in turn tended to offer the bottles to just one dairy in a marketing area. The Cream Top Spoon was usually given away  by the dairies to promote the use of the bottles.  This apparently was a very successful marketing strategy  but in the 50's  homogenization of milk became the thing, the cream line disappeared, and soon the whole Cream Top thing died.  Ah progress!!