I needed to attach a fitting to a 3/8" steel transmission line and then the other end would be to a 6AN fitting. I found this fitting from K-Motor. It allows you to install one end to a 3/8" steel line and it has 2 o-rings inside and then a clamp with a screw to keep the fitting from pulling off. The other end is a 6AN threaded end.

It lists this fitting as working with fuel lines but I assume it would work with a trans fluid line also. Anyone ever use such a fitting?

 

I assume that's meant to be used with fuel quick-disconnect lines. The retainer is a removable separate piece instead of being the plastic fingers on the inside. You'd want the end of the line to be perfectly formed, like those quick disconnect lines, and I don't know if the screw could make enough force to really hold it in place. Quick disconnect lines have a ridge that it would hit - not just smooth.
I'd recommend the compression fitting with ferrule to AN adapters. I don't like using ferrules on fuel - especially at EFI pressures - but for trans lines it's plenty good. One example is the Vibrant 16456

 

Correct, you need inverted flare to AN, like this http://RUSSELL Fuel Adapter Fitting ...a.co/d/h8QEL5y

 

So if the steel trans line end has a FLARED end, I could use that adapter I had in my original post.

But if the steel trans line has a SMOOTH end, I should use this adapter??

 

Pretty much

 

Not really.

What vehicle is this tube from? If it's a classic, then it's inverted flare on the end. More modern cars have used a wide variety of different types of fittings.

If the end is flared, then you can't really put anything right on the end. You'd need an adapter that goes from female inverted flare to something else, and such things don't actually exist. You can get male inverted flare to anything, but not the female side.

You can cut off the flared end so you just have a normal tube and use the compression tubing adapter (the one with the ferrule). Remember that ferrules are single-use, and they usually embed themselves into the tube. You can unscrew the tube side of that fitting, but the ferrule is never coming back off once it's been compressed.

It looks like you got this fitting:
https://kmotorperformance.com/shop/6...t-efi-adapter/
In the first picture they show a fuel line that's formed for a quick disconnect:

That ridge is what the red part grabs. The end of the tube has to be extremely smooth because it's just o-rings sealing it, and the ridge keeps the fitting from flying off the end. Also notice how the end of the tube is rounded over. That's to make sure the o-rings slip over the end and don't get cut by the tube. That fitting is really only good for a tube that has that type of quick disconnect formed on the end.