The top one is a 3/8 NPT to -6 AN adapter. The bottom is the old hard line that came out of my fuel pump... they are not the same. Both of them thread into a 3/8 brass fitting I found at a hardwaqre store, but the blue one will not fit into the pump.
What do I have here and where can I find an adapter from that to -6 AN?
The bottom one looks like the threads are worn out - maybe it started life like the top one?
edit: Is the top one really NPT threads? Now that I look at it again the threads look too straight for that.
It looks like the hard line had a flare. They make AN to inverted flare fittings:
https://www.summitracing.com/parts/rus-640380/overview/
When this happens to me I go to the local parts store and go in the back and start trying things.
Bottom one looks like a straight thread inverted flare nut, not NPT. If you don't have thread gauges and a dial caliper to figure out what thread you have you could take it to the hardware store/home depot/tractor supply and figure it out with their board of bolts and nuts thread checker.
What kinda application? My Toyota had this. (I'm told all Toyota have m14x1.5 inverted flare fuel fittings)
Camaro mechanical pump? If so rock auto says "Outlet Size 5/8-18 UNF"
Agreeing with above - not an NPT fitting, you can see the tube flare under the tube nut.
Now that I have looked at this on my PC screen that definitely is a flair fitting. You can see both the flared end of the tube in the fitting and the threads are strait not tapered like the one above it.