JB,
If you look at the top picture (your post with the 6-7 pics) you'll see a square piece (doesn't look like the same veneer as the cabinet) to the right of the ball tray. That is where the coin-op mechanism would go, if that table ever had one. At the same location, on the inside, there would have been a coin box and a lever that pushes a tray that releases the balls. I've never seen a DM home model, only the $$$ ones at playing locations, so yours may actually never have had a coin mechanism. The keyhole you describe actually sounds like the cabinet key which locks the top and bottom halves together. The padlock on the front is not original but is one reason I assume it once was a coin-op machine - don't know too many home units that need to be locked like that! My guess is that when the foosball craze from that period ended and vendors/owners were letting go of their inventory, someone bought that table for home use, removed the coin-op parts (maybe to repair other units??) patched the opening for the mechanism, removed the internal ball tray, and basically converted it to a home model. This is all speculation on my part and doesn't change my original ballpark price estimates from above. Those DM tables are relatively rare today and would appeal more to someone familiar with those tables from many years ago than someone wanting to spend the same money for a brand new, in the box table. You are looking for a pretty specific customer, and posting on this board was likely a good move!
Tyler