If you’re referring to the flat spot on the outer lifter body, from what I can tell, it is just a minor flat spot below the oil band. This allows some oil to leak past and go down to the lobe. I can’t find an off the shelf Big Block lifter with that, I figure a machinist that does lifter refacing may also be able to do it.
If I were to switch to a different solid flat tappet, I could use my current lifters after resurfacing or my set of OEM Mopar solid lifters that came with the ‘528 cam I bought in 2014. I’d have a machinist put a flat groove in those. I’m only guessing but it seems that any measure of increased oiling to the lobes is a good thing.