From foreground to background:
'87-'91 10-30 series. Dual frame mount holes for mounting above or below the frame rail. Uses single rubber mount for 'short foot' adapters.
'85-'86 10-30 series. Dual frame mount holes for mounting above or below the frame rail. Uses single rubber mount for 'short foot' adapters.
'81?-'84 10-20 series. Mounts below frame, uses dual rubber mounts for 'tall foot' adapters. Has more holes in stamping and has thinner flanges in areas.
'73-'80? 10-20 series. Mounts below frame, uses dual rubber mounts for 'tall foot' adapters.
'78 K20 series and other years unknown to me. Mounts below frame, plus has braces that tie into the top of the frame rail. Uses dual rubber mounts for 'tall foot' adapters.
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Not pictured are any '77-'84 K30 crossmembers. The only one I'm aware of is the one that HOTFOOT posted.
*Note*
The flat style K30 crossmember was also used in all the CUCV M1008. They did not get the same updates that civilian trucks got.