Keep very detailed records of your tests, JM. All the major lifter mfgrs have several models of rather similar lifters, often made in different countries to various designs, made from different spec steels & selling for different prices. Your very question has been driving established pro engine shops nuts for over a decade. And once we get definitive answers, it seems like someone goes bankrupt or there's a death or a 'merger' and we start over. I suspect Ford went to OHC decades ago for exactly this reason.
It eventually boils down to cost vs price. Someone recently said, 'racing runs on money, not on driver skill or horsepower'. That's true in the speed industry as well. Not trying to discourage your effort but you're not alone; realize the supplier you order your lifters from has the exact same problem. They literally cannot always get the same product twice in a row from big manufacturers, despite spending many thousands of dollars per year with them. And counterfeiters are everywhere. Some have fooled NASA with substandard parts that cost our space program millions. There are entire factories in the far-east that duplicate iconic shipping boxes and brand labels, to order!
There are only 2 (maybe 3) blank camshaft makers in the U.S. Guess how many specialize in the now-52-yr-old 351-C? EXAMPLE: mfgrs are advised of a 'break-through' in heat treating that allows them to do something desirable cheaper/quicker/better and under pressure from their stockholders, quietly change their product but not its p/n. End users slowly find while it once 'cured cancer', now the magic only works on Chevy cams but not Ford, or in iron blocks but not aluminum, or it depends on a particular oil or additive that itself regularly changes without notice due to constant gov't (pollution) decree.
Join <www.speed-talk.com> (free- only takes seconds). Like the DeTomaso forums, this is a series of engine builder forums populated by tech-reps from major cam grinders, piston mfgrs, NASCAR race engineers and pro engine builders as well as one-motor-per-year street builders like you and I. Some are in Germany, Sweden or England. Drag racers, roadracers, motorcycle builders- all have questions. Lurkers are welcome. Many of the guys on the forums participate in yearly public dynomometer 'engine master' contests that are reprinted in popular magazines. At least two regular posters are judges of such contests.
These forums also post pertinate engine news and have enormous archival sources for new people (and pros) to ask questions and get real answers. I get many of my own tech answers here; sometimes I find there really IS no answer. But all us go-fasters still have hopes for doing better tomorrow. Good luck.