The "gussett" is like a small reinforcement bracket. The earlier cars didn't have them, and MB started seeing them crack, so they came up with this reinforcement idea, which works pretty good. i would say your car probably had them from the factory, as soon as they came up with this idea they added the bracket at the factory. I don't know if they will stand behind it if the bracket (gussett) was there from the factory, they only seem willing to help on one that was built before the modification was done at the plant. Seems more than fair from the standpoint that the bulletin and letters were sent out back in the mid-80's, now here it is 15+ years later and they paid for a new subframe for some guy who never had the gussetts welded in. If he would have had it done back then, it'd probably never cracked.
At this point I would get it inspected at the dealer and see if there is anything MB would be wiling to do. i'd be curious to know if the gussetts were installed at the factory. I may be able to fish up that recall bulletin, MB DOC is probably right that an 83 is too new to be affected by the old recall, but I'll look. You can also ask the dealer to check VMI to see if there is any outstanding recall work on the car, if there is one for this repair, then I'd say you have a new subframe coming!