Perry Scheuerman

I believe Ed Hawkins some years back posted a spreadsheet showing the freight equipment renumberings. The group of cars you referred to, MP 3900-3049, were renumbered MP 700600-700649. It appears that there was a sister set, MI 6150-6199, renumbered to MI 700450-700499. Both groups were 29’3″ IL, 2003 cu ft. cars built in 1957.
I don’t know the renumbering date. Hopefully the spreadsheet will be migrated over from the old MPHS site.