Ok battery has arrived and I've tested it with 2 differently-branded 4.2V/420mA chargers (of course they don't have a brand stamped on them, but they look a bit different on the contacts and the sticker, so I guess they're from different factories).

With both of them the battery fully charges to exactly 3.7/3.73V.

I must add that it looks like the output is fixed, anyway, as in the internal circuitry keeps it at 3.7 during the whole charge-life, but I'll have to test it (depleting this battery may take long, fortunately :) ), so it's not like any other cheap 3.7V battery that can't keep 3.7 under load and it slowly decreases while discharging.

I'm going to do some more tests and report, but it's probably a 3.7V battery. Maybe it also depend on different batches, as mine is slightly different than the one in the photos here.