Beginning in the mid-1960s many brands of travel trailer (including Airstream) used Thetford waste valves. Although Thetford valves are rarely used on new trailers, some of the parts are still available. Thetford valves have four bolt holes at 4-1/4" on center.
Attn Airstream owners: If your trailer is 1960 to 1963, you do not have a Thetford system. Your waste fittings are an obsolete standard. We have adapters here: Early 1960s Waste Adapters. If you have a 1964 Airstream you may or may not have a Thetford valve but you do not have the standard Thetford bayonet. If you are 1964 and do not have a Thetford valve, you will need our adapter mentioned above.
If your trailer is earlier than the 1960s, will need to start from scratch. Sadly, there are no valves or adapters available.