I looked at JB's xls over on the FB site. JB's xls has 2,939 observations. Here's some data on low count Model/Chamberrings from his xls. He does not have any 99E 250-3000, but we did see one 1960 here a couple years ago.

Attached Images
Low observed chambering after 1959.jpg (36.56 KB, 67 downloads)