Comment by self_awareness

If the sender requests GPLv2, he should receive GPL version 2.

If the sender requests GPL, I find it natural for him to receive version 3, because it's the latest version. At the time of receiving the license, he gains knowledge about the existence of version 3 (the header on the print says the GPL he received is version 3).

If the sender has a notice about GPLv2, it means that there's a high chance that there's also GPLv1. This should be a sufficient hint that requesting only "GPL" is not sufficient, because the sender should be aware of the risk of receiving GPLv1 if he won't mention the "v2".