Found the super dry and crusty area whilst cleaning the trigger group! Doesn't appear to have been apart in ages.
Did a minor soak with Hoppes.
One of the very few "A" marked parts in the entire rifle. At least it's easily enough identified now, as if the rough machining wasn't a giveaway!
Also realized whilst doing a tung oil rub that the handguard had not been relieved at the rear right corner. (Lower left in the photo) Rather happy about that!:
Not so shiny now as when photo was done. Was in the middle of application when the realization hit.