The following is culled from the Mercedes-Benz Service Technical Data Passenger Cars books, April 1985 edition, December 1988 edition, and October 1993 edition.

OM615, 616, 617: T-stat begins opening @ 80 +/-2 deg C, minimum stroke 8mm @ 94 deg C

OM601, 602.91, 603.91: begin opening @ 85 +/-2 deg C, minimum stroke 8mm @ 94 deg C

OM602.96, 603.96, 603.97: begin opening @ 80 +/-2 deg C, minimum stroke 8mm @ 100 deg C

OM604, 605, 606: begin opening @ 85 +/-2 deg C, minimum stroke 8mm @ 100 deg C

The bypass disc diameters, which may indicate compatibility of thermostats with multiple engines (if, for instance, you are tempted to try a hotter or cooler thermostat):

All OM60x: 40mm (looks like some options are available here)

All OM61x: 37mm (sorry, no soup for you!)

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC