A couple of the replies have mentioned it - but to emphasize: the odds are 10-to-1 that it is the seal between the pump and the head. Then the leaked coolant "wicks" its way backward along the head/block joint to imitate a leaking head gasket. If you can manage to find a shop with a cooling system pressure tester that will fit the cap on your reservoir, great. That should enable you to track it down.
The water pumps on these cars are pretty much a maintenance item, so if you don't know when last replaced, do it now. You are about halfway into the labor of a timing belt replacement at this point, so there's another might-as-well.
I REALLY don't like the "Stop-Leak" type of products. Too many stories of heater cores that worked OK before, not at all after.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)
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