[jimmontg]

Try tempering at 435. I always tempered L6 at 435 for TWO hours and that gave me Rc59/60 and I used an oil quench. It was still a little brittle, but the edge wouldn't chip. In other words standard for a knife unless it's deferentially ht. The quenching oil we had was a slower oil, it was a Chevron made oil and was bought for O1 and other slower quench steels. I couldn't give you the specifics on it other than it was a higher viscosity oil. Actually Parks 50 is too fast for O1 and other alloys like the S series shock absorbing steels. L6 also takes a slower oil too. That's why canola oil works well for these steels as does automatic transmission fluid as they are thicker, just avoid any smoke or fumes from ANY quench oil. By the way, canola oil goes bad if used often. Takes about a year and you'll start getting inconsistent results is how it starts. This is just something I have heard here on knife Network.

Now lets take a look at your results.
Even the bright orange grain looked fine, good.
Every thing was brittle, indeed your temper is to low and too short, two hours is standard Rasmus and don't worry about going too long, it's about temperature more than time except the two hours or going too short.
The one beveled coupon didn't warp, good sign that you have an alloy steel versus a plain 1075 steel. How long was the beveled edge?
The brine quenched one would be the most brittle as it quenched the fastest. I wouldn't count on a brine quenched knife not cracking based on a single coupon 3.2 mm thick.

Go ahead and grind out a knife and and coupon, then take the bevel down to within 0.5 to 1 mm on the edge, heat to above magnetic for a minute and quench in warm oil like 120 degrees. Temper for two hours at 435 and start cutting and finally see how much flex the coupon has.
Another test is if you have any ferric acid or cold gun blue like Birchwood Casey Super Blue. Put a few drops of the acid on it and wipe off. If it doesn't darken then you have a high nickel steel. If you can't get the acid try the gun blue and if it won't blue consistently, but is very blotchy same as before, lots of nickel. If it air hardens under an air blast and is brittle you most likely have L6.