Bauway 510s are either 2.5 or 2.8. I have seen vendors claim both but I don't think there are two different ones. The SLB SR is supposed to be 2.8 but I have now purchased them from two different vendors and they meter at more like 3.2. Supposedly they drop with use. They do provide a nice
vape at 5V though, even when new and they are cheap.
The guy that runs KVS uses the Bauways on his personal Big Kazoo but I have not tried one yet.