A generic R50-series receptacle for fine-pitch probe fields, with a solder-cup termination. It is pressed into the fixture's probe plate and the spring probe seats in it, so a worn probe can be pulled and replaced without unsoldering or rewiring the fixture.
The R50 body is the smallest of the generic R-series, for boards whose test points are too close together to lay out on a 75 mil field. This is a solder-cup variant; within the R50 range the numeral marks the body length, so check the overall length against your plate stack before substituting one R50 for another.
Generic R50 series, for fine-pitch (50 mil class) test-point spacing
