What is the small sliding arrow/pointer on the rim of this 2 5/8 S W Greenline gauge called, or are they available--Part number?
Not really...a "tell tale" is something that the gauge pointer contacts, and moves to the highest reading.
As was suggested you could probably make some out of SS that would be better fitting to your particular gauges, the back bend would have to be sharp and shallow so as not to go too far past the bezel and scratch the dash. Yeah, big help.