The LDR board is the signature correction board used in the G.I. - Motorola line extenders and is factory set to enhance optimal frequency response flatness. It is field tunable and should only be adjusted by an experienced technician. The MDR board is used in the mini bridgers. They are not interchangable but will fit in LE or mini bridger modules.
This is CABL.com posting #296869. Tiny Link: cabl.co/mbpon