I'm going to guess that it's a 615 engine out of a 220D.

I think this because of the 61501080 number you gave and finding that 220D's of the early 70's had those according to this http://home.hiwaay.net/~gbf/mbmodels.html

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k