FYI- just another point of reference. I just tested my 300SD W116 w/125k miles (617.950) and it was 390 to 410 across all 5. I would think yours should be in that ballpark as well. I can't seem to find your engine nominal compression numbers (Braingears has your manual) but I think you have a squawk against your seller.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)