Kevin is correct that the rubber piece is called a "grommet".

More details from the 1929-53 Chevy Master Parts and Accessories catalog:
Parts Group 9.775
47-49 COMM/UTIL (exc. D-D) GROMMET, throttle, choke, heat, oil gauge pipe thru dash, rubber (1 5/8 O.D. - 4 holes) GM# 3684771
Another source for a reproduction is
1947-49 Truck Firewall Grommet 4 hole.
It can also be found as one piece of the
1947-49 Truck Firewall Grommets Kit 6 PCS if you are needing more than just that one piece.
If you are also looking for the metal "RETAINER, Grommet" GM# 3653786 (not sure if the picture you posted is of your truck or just a sample showing what you are looking for?), I couldn't find that piece being offered as a reproduction or even used by a web search.

One ray of hope though if you need to find one of the metal retainers, they were also used on ALL 1940-48 Chevy passenger cars. Could be a donor vehicle out there near by???
If you really want just a generic rubber grommet for pass through protection and aren't worried about having the metal retainer, this
4 hole grommet listed for 1947-59 Trucks maybe all you want. That grommet is self retaining and doesn't require the metal retainer.
Hope the info may help you out.
