This whole time I’d been assuming that both these SKUs of ECC DDR memory were equivalent. Rod Bland sorted me out at RamCity:
DDR4 modules can optionally [b]e “registered” (“buffered”), which improves signal integrity (and hence potentially clock rates and physical slot capacity) by electrically buffering the signals at a cost of an extra clock of increased latency. Those modules are identified by an additional R in their designation, e.g. PC4-19200R. Typically modules with this designation are actually ECC Registered, but the ‘E’ of ‘ECC’ is not always shown. Whereas non-registered (a.k.a. unbuffered RAM) may be identified by an additional U in the designation. e.g. PC4-19200U.
It never occurred to me that -R stood for registered/buffered. The Supermicro board in my new homelab machine only showed unbuffered memory compatibility, so I guess it’s just luck I didn’t accidentally pick up the wrong type.
ServeTheHome also has a great article.