The one we use is made by Pannier, out of Pittsburg (www.pannier.com).
We are currently using 3 x 5 steel tags which is large enough for all our information in a reasonably large font. The operator enters the information on a keyboard, lays a tags on a magnetic strip, and pushes the start button. Our machine has an air powered stylus that is adjustable for stroke depth, and it sounds like a fast sewing machine in operation. The cost of ours was around $6000, and you can get various models.
The reason they have worked well with galvanizing is the characters are deep and wide and if you can't read one side you usually can read the other, although a mirror image. The tags are not indestructable, wire them on where they are less likely to be damaged.
We had also used another type of machine that resembles one of those hand squeeze plastic label makers. I don't remember the company, we found it by doing a web search. It used an 3/4" wide steel "tape" in roll form and the characters were about 1/2". You turned a wheel to the character you wanted and pressed down on the handle. Those tags worked well also. Unfortunately, most people felt a pipe was needed for leverage- seems like they felt better when they could feel an extra "crunch" at the bottom of the stroke. The embossing wheel and other parts didn't last. (That's why we went with the Pannier- because heavy hands weren't needed and we limited the number of people who can run it) Cost was about $2500.
Hope this helps,
CHGuilford