The accessory valve that was spec'ed by Chevy in some installations was installed on the thermostat housing, as Tiny has done. I'm not sure that covers the full range of factory fitment locations and I'm guessing does not.
The first image is a figure from the '53 Chevy accessories installation manual (a reprint copy). Of several heater installations shown in that manual, it is the only one to show a shut-off valve, and is in the section for only one heater style used on passenger cars. None of the truck installations in the '53 manual show a shut-off valve. Regardless, seems a good idea for systems that don't otherwise have a mechanism for complete shut-off of coolant circulation through the heater core.
The valve (second and third images, same unit Brad posted) was used in installations like the one shown in the accessory manual figure, and is a fairly compact, bronze gate valve, threaded with the correct 3/8" NPT for direct install in the thermostat housing, replacing the hose nipple that would normally be used at that position. I've seen NOS units on eBay ranging from about $5 to over $100 when marked "Corvette." Same valve. The original part, Group # 8.870, PN 985.533 was superseded sometime over the years. There were two part #s listed in the '68 Master Parts Catalog, Group 8.866, PN 3132607 with 5/8" hose connection or PN 3741428 for 3/4" hose.
