Most likely if it's a sawmill blade from the 80s it is 15N20, which would heat treat about the same as O1. L6 is a bit different, but not by a lot, get it up to 1500-50 and use hot oil around 130. 15N20 would be too hot at 1550 is only difference, but with 2% nickel the grain growth wouldn't be too bad. Matter of a fact 15N20 HT about 1480 being it's quench temp. Alpha Knife supply has simple HT instructions for 15N20.
https://www.alphaknifesupply.com/zda...eelC-15N20.htm
L6 has 1.7 nickel but has almost 1% chrome and .35% Moly in it depending who makes it, it may or may not have moly in it and the nickel can be lower as well.
Oh temper temperatures are a bit lower than O1 for both steels. About 350 for 15N20 for RC 60 and 425 for L6 for 60, I shoot for RC 58 to 60 with these as the nickel imparts toughness so brittleness isn't an issue. I don't see the point of going much past 60 hard as most people can't sharpen it unless they have a diamond and a machine or know how.
http://cintool.com/catalog/Oil_Hardening/L6.pdf
They are both forging steels Justin and if it's 15N20 a treasure as you can't get that stuff any thicker than 1/8" as it is mostly used in Damascus for the shininess it imparts because the nickel resists the acid etch, but it makes a great knife by itself. Matter of a fact if you etched a piece of it and it stayed shiny then it's 15N20 or high nickel L6 for sure in which case your HT is fairly simple other than hammering it thinner than 3/8" thick.
Heck they used to order bandsaw sawmill blades out of 1084 with 2% nickel added. A forging friend of mine up in BC Canada has a bunch of it 3/16 or 1/4 thick, I don't remember.
Ray lives in the PNW in Washington so he has run across this stuff from time to time I'm sure.