As I drill down, perhaps as verification of a single line sending unit (no return)............ There are quite a few variances for sending units.
http://www.gmpartswiki.com/getpage?pageid=21463. Here's what I found on parts WIKI for 71 C-10. All single line sending units. A few of these probably don't apply:
67-72 6428272 Service 10 (D-D) Meter assm. W gasket Single Line Sender: Don't know anything about vans, but does this mean service 10 van only, or service 10 & van?
68-71 6428271 Service 10-20 (single unit) (327, 350, 396) Meter assm. W gasket Single Line Sender: 3/8" line.
69-71 6428361 Service 10-30 - W/4BC -350, 396, 400 Meter assm. W gasket Single Line Sender: I believe 4BC means four barrel carb. Looks to be the correct sender for my 71 402 4BC. 3/8" line.
70-72 6428850 Fleetside Meter assm. Aux. Fuel Tank Single Line Sender: Looks to be for an aux. tank. Unsure of configuration.