When I assigned Item Lookup Codes using A for auto-number or AC for auto-categorize, the component items end up with a prefix of Axxxxxx or ACxxxxx.

You will need to renumber the component items from Items, Database. Select the first component entry using Lookup, double-click to view Item Properties, enter AC (or A), then hit Enter to renumber that item. Repeat for each component item in the matrix.

WARNING: You should NOT set the default search to Find if you want easy access to your Items database. Find results will not show matrix class items or assemblies for editing and maintenance. If you have set the default item search to Find, go to Manager, File, Configuration, Options, Items and uncheck "Always display Find...".