MR> While adding an old drive to one of my machines for the purposes of
 MR> setting up a bunch of HPFS partitions to test my HPFS programs, I
 MR> decided to first create four primary partitions, and change the
 MR> partition types to 0x06.
 MR>
 MR> I then booted from OS/2 floppies, and the first one in the partition
 MR> table (which FDISK strangely made the last partition on the drive, in
 MR> terms of sector offset - they were all ordered that way) was the only
 MR> one assigned a letter.

It's a pity that you didn't try MS-DOS as well whilst you were at it, just to
confirm my educated guess (based on the fact that prior to MS-DOS version 3.3
one was forced to use multiple visible primary partitions for hard discs
larger than 32MeB, since there was no such thing as an extended partition)
that it, too, alongside Windows NT, FreeDOS, linux, and DR-DOS (all of which I 
have either seen do this or have traced through the source code for), can also 
see multiple visible primary partitions.

 ¯ JdeBP ®

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