Gen 1s are very easy to identify with their smooth, pebbled grip including backstrap and frontstrap. IMO, that's more definitive than a serial number, but I suppose a Gen 1 frame could be replaced with a Gen 2, especially if it was a police gun. (Glock would change out the frame for a nominal charge back then.). So, you could have a Gen1 serial numbered gun without the gen1 frame...
If you still have black parts, though, I rather doubt that's the case, since Glock would have changed out those trigger group parts for free when they saw them. If you want to shoot it and changing trigger parts to non-original is on the list of changes, I'd get a trigger parts upgrade kit and sell the old parts here to a collector.