Still haven't figured out all the variants. 
187-207  8550 MODEL R21
187-049  IBM PS/2 Model 50 (-021)
188-080  Model 50 Z  Systems w/Improved HD and Memory ( -031,-061)
 

My pocket ref sez:

System  FRU      Size Int           IBM
8550    72X8522  20   ST506         WD325N
8550Z   6128287  30   ST506 "like"  WD336RT
        6128294  30   ESDI "like"   WD387
8570    56F8895  160  ESDI "like"   WD380S
 

8550-021 - 20MB HD (uses 72X8505/90X8642)
8550-031,-061 160MB (used 90X9441, no chips) 50Z
8550-032 - 30MB 50Z
8550-061 - 60MB 50Z

Dave Beem spouts off with:
   Apparently it is a different drive (50-pin edge connector) than the 20Mb the Model 25/30 (44-pin) had. The Model 25/30 type also had a 30Mb brother that ran on the 286 version of those PS/2s. On the Model 50 the 20Mb was the only ST-506 drive type.

David Beem sez:
   90X9441 is ESDI. There are two circuit board variants of this, with no disernable differences: Circuit board 15F8434 has silkscreen outlines (not populated) for two capacitors & wider power runs than the 90X9354 circuit board.

90X9571 PN 15F6993 (50-021 w/60MB upgrade)

J1 72 pin edgecard socket
U18,19 BIOS

Per Mueller:
   "In response to user complaints about the storage capacity of the original 20M Model 50, IBM offers the PS/2 60M hard disk drive as an upgrade option. You can install this drive by replacing the existing 20M [ST-506] hard disk in the PS/2 Model 50 (8550-021) or 30M [ESDI] in the Model 50 Z (8550-031).
   No trade-in is available for the earlier drive. This drive provides 60M of storage and a faster access time of 27ms. The replacement adapter card required for the 50-021 is included [but is already there on the 50-031]."