I am thinking perhaps a very large batch. When I rebuilt my 1971 B20E the lifters had serious spalling on the contact face with the cam, so Volvo 'fixed' the problem some time after 1971. For what it is worth, I have read that the replacement OEM style lifters do not suffer the same problem. But, that is Internet wisdom - for what it is worth.
As to John V.'s comments, perhaps an option if you are starting off with Volvo's later lifters and you have access to a machine shop knowledgeable about solid lifters and rockers and that kind of stuff. That is becoming an increasingly rare commodity. At $8 for a resurface job it is cheaper than a set of the lightweight lifters and matching push rods; but, not a huge saving considering the overall cost of an engine re-build. However, original style lifters are available for $7.75 (VP Auto) so the only saving is the shipping cost (compared to a local machine shop) and if you plan ahead and get parts together then perhaps no saving at all.