I'm going to stay away from the, "what does the last two digits really mean," discussion. What I do feel I can comment on is the fire grate and what lasts well. I find using 3/4"x1"+ on edge works really well for sag resistance. Having the extra steel in the air stream compared to the smaller cross section in contact with the coals keeps the whole cooler and being deeper with the coolest section at the bottom is more structurally sound. Basically it lasts a lot longer without sagging or oxidizing.

Frosty the Lucky.

Having seen several railroad rail grates that were sagged and burnt up, I believe that all such items are "consumables" and so should be expected to have a limited use life with the cheap to build vs how long it will last being the controlling factors. Now a platinum alloy one....

Glanced through one of Ruoff's Mat Sci texts, and the Metals desktop manual and the 1948 ASM handbook over breakfast.
The latter two gave charts of the compositions of the ANSI and SAE designations alloy contents but did not address the numbering scheme per se. (got pulled into the 1948's discussion of "alloy wrought iron" which I don't recall seeing in my more modern editions...) Still looking but I'm quite busy this weekend with folks stopping by Saturday and Teaching at NM Tech Sunday--just had one of the advanced students asking me if he can make a kunai...

Digging up old material but figured better to post here since its most related to my question.  I am making a fire pit.  4 sides and bottom with legs. Is 1/8" Cold Rolled thick enough for such a project ?  I see them being made out of 3/16 and 1/4 but this one is a break down model for transporting when going off road. Want to keep weight down.  Any advise appreciated.

If you have a thick layer of ash insulation, 1/8" might work. Otherwise, that thickness probably will warp on you if the fire is directly on it.

Or you could reinforce it with some sqare stock or something.

It is going to warp. Accept it and take it into consideration in your design.

Six sides, or round(ish), and 3 legs will work better than 4 at the corners. Campsites are seldom level anyway, and three legs will always stand up.

Tabs and slots on the sides, with wire or nails to peg the slots while in use.

Here is a simple four sided design that I found online.

Use the tub out of an old washing machine. Lightweight, sturdy, and can be used for holding other things when being transported. And you can get them for free many times. There are even some out their now that are stainless.

Thanks for the responses.  I went ahead with 1/8 inch and will likely make a separate, replaceable bottom that stands off and takes most of the heat or weld reinforcement strips along the bottom, or maybe both. This uses tabs with holes and pins that insert into the holes while in use. Design I found online and had to modify some. Going to make a bottomless version as well.

I like how that incorporates the old and new grill designs.