Resolved.
When copying a simple installation's HIERARCH.PM file to a new installation, pmail will re-assign the unique ID of the primary mailbox. It is a simple matter to do a find and replace on that unique ID.
Example:
1) Delete the HIERARCH.PM file from the new installation, restart pmail, exit pmail and then look at the first line of the file, which will be something like:
2,1,"5F242154:My mailbox","","My mailbox"
2) Copy the old HIERARCH.PM file into the new installation. A few lines from my old HIERARCH.PM file look like this:
2,1,"56A33625:My mailbox","","My mailbox"
1,1,"1F533666:Tray1","56A33625:My mailbox","Tray1"
0,0,"1CF29095:0EFA:FOL132AA","1F533666:Tray1","Folder beneath Tray1"
0,0,"0E562976:3863:FOL031F0","56A33625:My mailbox","Folder In Mailbox - Not in Tray"
3) You would then find and replace each occurrence of 56A33625 with the new installation's unique ID for the mailbox (5F242154).
4) Save the HIERARCH.PM file into the new installation and restart pmail.