Is the m die any different than just using an rcbs expander die with custom mandrels or does it do something extra.

Lyman M die rods are threaded on the end so you can quickly change expander plugs. Unfortunately Lyman sells the expander rods with plugs as an assembly and not just the expander plugs. But they're pretty cheap, and I just set the rods aside and use only the plugs. I drilled a wood block with a bunch of holes and used a sharpie to write diameters by each expander.
I buy the expanders direct from Lyman and since they don't sell custom sizes I just buy the next size larger and turn them down. Chuck the threaded shank in my drill press and use a file to work them down to the exact size I need for whatever I'm expanding. They have a large variety of sizes for sale, but occasionally they're a bit larger than I need. Last I bought were less than $4 each, so I bought extras in .45-70 and .38-55 to save on shipping.

Just checked and I do have one set of 45-90 Lyman dies. The expander die is stamped M-2. So it must be the M die. Lyman did good. Slick. I've always made my own when I needed them. That's why I told Jack to get on the lathe and make one. Cheaper thata way. As Johnny Cash would say...