The Weber Genesis grills from 1998 use a quick-release style regulator, which looks similar to an air compress quick disconnect fitting. Tanks that use these adaptors no longer exist, are hard to fill, and are reaching their end of life in terms of safety.
Weber Genesis grill:
Start by wrapping the 1/8" NPT side of the adaptor with pipe tape:
Locate the old regulator hose where it enters the burners:
Remove the old regulator and hose from the burner rail with a small wrench:
The regulator and hose are removed:
Insert the new adaptor by hand, being careful not to cross-thread it:
Use a small wrench to tighten the adaptor, be careful not to over tighten and strip out the threads:
The adaptor will go in about this far:
Attach the new regulator to the 3/8" flare nut. There is no need to use pipe tape on this end, the flare nut will make a tight seal:
Test for gas leaks with soapy water.