Great point and something people lose sight of.
Piper Navajo: 1967
Cessna 402: 1967
BN Islander: 1967
Beech Queen Air: 1960
Beech Baron 58: 1969 (a little on the small size to be considered an airliner, but I can find reference to a few small commuters and air taxis using it)
Partenavia P.68: 1971 (barely used as an airliner)
Every other piston airliner is even older and a barely postwar design.
The last one Partenavia P.68 was developed into a 9seat TP now on offer as the Vulcanair A-viator
(AP68TP-600) EIS 1986. Wiki: Partenavia P68
Funny fact Partenavia is a preceding company of both Tecnam (P92) and Vulcanair (P68). Tecnam moved into the P68 market with the P2006 and P2012.
I expect a retractable landing gear will be an future option for the P2012. And I think it's a nice addition besides the single TP 9 seaters (Pilatus P12, Cestna 208, Kodiak) and besides the BN-2.
Edit: Thanks for posting the pictures PartsGuy20.