I'm looking at the check valve in the parts catalog for R/W body. Both big block and 360 uses it. The preceding page has H/F/N/G oil cooler w/ E58 small block engine, and you're right, no check valve. The implementation is basically the same except for that, fed via the upper gallery in the back, to a return in the fuel pump bolt.

If anyone wants it, the checkvalve PN is 2264514 (an old number).

A search comes up with this description: "Disc, oil pressure relief valve." At this point I'm wondering if it uses a stiff opening spring and closes off flow in the event of low pressure (to prevent starvation). Mine is around somewhere, I wish I could find it to take a look. I would love to know exactly what they were doing with it. I'd almost bet money it's just a simple check valve, but it's hard to find descriptions of minutiae like this