MRU is not a good source for the information.

The PST files are stored in the messenging subsystem registry entries, but they are a little tricky to locate as they are binary objects.

Have a look at the profiles under HKCU\Software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles

You will need to decipher these but for example look for the subkey 0a0d020000000000c000000000000046 and then take a look in the binary entries. On my machine 001f0324 has my primary archive PST.