If I remember correctly there are two or three part numbers for Comp solid flat tappets . I can look them up for you if need be. I have used both styles with good success ,even with more aggressive lobes than the 590. I use Driven Joe Gibbs) assembly grease and break in oil.
Just pulled the one piece Comp lifters out of my old 440 today. I have well over 100 dyno pulls on them with Comps MM lobes and the cam and lifters are perfect.
Guys switch to rollers for safety but the extra expenses will buy a lot of good oil.
Keith