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 with Improved Hardfile and Memory ( -031,-061)
188-082 IBM PS/2 60MB FIXED DISK DRIVE

8550 ST-506 (50 pin edgecard) 

8550 DBA-ESDI, Bare (72 pin edgecard, Bare card)

8550 Drive Controllers (Peter Wendt)

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

60mbbuff.exe  PS/2 60MB Fixed Disk Buffer Test v.1.10 
60mfdisk.com 60MB Fixed Disk Upg for M50-021 V.1.00

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

   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.

6127881B 

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

90X9354 (90X9441)

8550 90X9571 P/N 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]."

8550 Drive Controllers 
Peter H. Wendt     1/28/09

There are 3 risers from IBM:

- Bare ESDI riser for the Mod. 50Z to be used with the 72-pin card-edge drives. Passive. Does nothing but guiding signals.

- MFM controller for the 8550-021 with the "narrow" card-edge slot for the 20MB MFM drive. ID is DFFD, just like for the full length Mod. 60/80 MFM controller.
(http://www.mcamafia.de/mcapage0/jpg/dffd_2.jpg)

- ESDI upgrade riser, with the 72-pin card edge slot for the drive and some glue logic to make drive addressing possible, since the original Mod.50 lacks the ESDI code in the machine BIOS. Controller ID is also DF9F, which is that of the attached drive, which actually *is* a MCA controller card + drive mechanism in one. Back then ESDI was a big thing and AT only knew MFM (not even RLL). The early Mod. 50 BIOS is largely that of the AT ...

In addition I saw a 5.25" FDD controller riser (RFI ? Long gone), a variant of the ARCO IDE card (without rear bezel and handle), a similar thing from Procom for a 3.5" drive with a special sled (IDE most likely), a QIC tape adapter (Borsu or Iomega - can't recall), which feds a flat ribbon cable  out at the rear. Unit was a bulky 5.25" drive with power supply, fan and impressive power switch. And another tape adapter which operated with a floppy streamer in the second FDD bay. All these adapters were for the 50Z only, since that one did not need an extra HD controller - unlike the original -021 (non-Z).