Although Gerber has used ATS-34 in some of their knives in recent years, I haven't seen it in the Mark II yet. I do recall reading a couple of years ago that they were starting to forge their Mark I and Mark II blades. A long time ago they also made some knives with M2 blades.
Its my understanding that Gerber used M2 high speed tool steel in their MK1 and MK11 daggers up until a certain point(early 80s maybe?), and from that point on, have used 440A. I have a MKII that I bought from the USMC exchange in 1977 or 78 and it is definitely not 440A.
I have read in older Gerber literature where L6 was used on the
Vietnam war era Mark-type knives and M2 was used on the high speed tool steel hunting knives. The hunting knives then were hard chrome or electroless nickel plated. I have several of their older high speed steel hunting knives and they are excellent edge holders.
The older Folding Sportsmen line and Magnum folding hunter were 440C until the early 80's I believe. They are also great old folders.
IMO, When Pete Gerber sold and left the company the greatness left with him.
The older MKI and MKII were made from L6 (sawmill steel) and at some point in the very late 70's they switched to stainless.
One way you can tell what you have is by the serial number. I have found that the L6 blades had only numbers in the serial. The stainless blades contained an "S" and many times a lot letter like B55678S.
The earliest stainless used in these knives was 440C...then 440A....according to my collection of past literature from Gerber.
L6 was the best but had higher maintenence to prevent rust. It was tough and held a tremendous edge which gave these daggers much more utility than their design. When they switched to stainless, the MK series became little more than just daggers.