Dornier Do 217 M - the Luftwaffe's best bomber

Currently at koelsch333's Ebay sales courtesy of Marco is this nice set of Do 217s including what may be an M-4 GB+CV on the compass-swinging platform at Rechlin according to the caption. The 'M' series was the DB 603 in-line engined variant and Rechlin log books from December 1942 to May 1943 included frequent references to an 'M-4' apparently used for de-icing and cabin heating tests as well as high-altitude testing of the DB 603s.  The 'N' variant was also tested with DB 603 engines at Rechlin as seen in the final image in this series.