The round ones were a less expensive accesory, so I'd bet they were more commonly bought.
The recirculating heater core is
$289 (Jim Carter) and it needs to be installed/soldered (
$300 for this core/motor/plumbing replacement). There might not be a 53-55 "deluxe" heater core available - the 47-52 core (ready to install) is $170.
