Why would you think it didn't happen very often?
As previous replies mentioned it is a mid-air refueling using a drouge basket and probe type system as is often used with helicopters today as opposed to the what most folks are more familiar with which is the flying boom.
A couple of major differences between the type systems are that the drouge is often used off the wings and the receiving aircraft has to fly their probe into the basket. Whereas the flying boom requires the receiving aircraft to maintain the proper relative position and that boom operator controls the position and extension of the boom.
I saw somewhere recently one of the civilian companies that provides in air refueling services is refitting their aircraft to have both a boom under the fuselage and drouges under the wings thus increasing their flexibility and increasing the situations in which they can refuel more than one aircraft at the same time.
Did I win?? Do I get a prize..........................
What I meant was its fairly unusual to see a Marine jet fighter refuelling from a KB-50. I seriously doubt that it occurred very much.
Do you have information which indicates otherwise?