There are 2 numbers in the parts books;
for "first type" cabs 2039428
for "second type" cabs 594498
The above seem to apply from C8 to C60, but the C60X and other GM vehicles with the 270 cubic inch engines seem to have used (more than one) different caps.
One online vendor lists the cap for the "second type" cab as
http://www.parts123.com/PartFrame.as...vrolet_Part_Co
They also cross reference to the following part numbers (without saying whose the part numbers are): 33001, 703-1400, 850901, RC10 and RC68
Another vendor at:
http://www.chevsofthe40s.com/Shop/se...talog=RO&go=go says the RC-10 is an AC Delco part number.
I'm sure other vendors have the caps too.