That's the seal like Robyn said. There's a catch on that though, there's 2 thermostat housing casting types with different depth "depressions" for those. The deeper one is the type with the rubber seal, the shallow one utilizes the old-school gasket. A lot of parts systems/books lists them as interchanable, but in reality they are not. The Made in China thermostats are junk IMO.
Don
1993 6.5 3500 DWR Ext Cab
205,000 miles, GM-8 turbo, 3" downpipe,
straight exhaust to duals, '97 Airbox,
fabb'd mech boost controller, 1/4 turnup on 4911, triple gauges
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