None of the exhaust manifold bolt holes are open to any waterway in the engine, so the leak can't be coming from the missing bolt - incidentally, on the 124 Spider, the exhaust manifold is usually held on by studs, this is better on alloy cyl. heads as the threads are less likely to strip out.
As regards where the leak might be -
1) there is a large cover plate that closes off the coolant jacket, held on by 6 x 6mm (10mm wrench size) at the back of the cyl. block just above the crankshaft.
2) there are 4 frost/expansion plugs (40mm diameter) fitted into the coolant jacket on the exhaust manifold side of the cylinder block.
3) there is a metal return pipe from the heater that connects one of the heater hoses to the back of the water pump, this runs just under the exhaust manifold and has a bracket that attaches to one of the (possibly the rearmost) exhaust manifold studs or bolts in your case. This pipe can wear through if the bracket is loose or was left not attached leading to a leak, or the pipe can rust out from the inside if anti-freeze/corrosion inhibitor wasn't used.
Quickest way of locating the leak would probably be to perform what is usually called a "coolant system pressure check" - this uses a hand pump and adaptor to fit the radiator to pressurise the coolant system and locate the leak.
