That's not an SB2 frame, it's a 1976 cast iron frame, there's a matte nickle frame with the same kind of voids, a lot different than an SB2 alloy frame, see the pics in the FAQ.
There were no SB1 or SB2 frames made before the end of 1986, they had an entirely different SN prefix, prior mfr were all cast iron.
Tim