I have been pondering this for some time now and I'm sure others have, though I've not yet seen posted.
To begin: We all know about C-3PO's mind wipe and why this was done and we have also discussed why R2D2's mind wasn't wiped. I would now like to address something obvious to question: Why doesn't Owen Lars seem to remember C-3PO when purchasing him form the Jawas? I would figure, with Threepio being such the distinct outspoken droid he is, that Uncle Owen would recognize him as being, or being very similar to, the same protocall droid that came along with that Shmi lady. With Owen being so adament about keeping Luke safe and preventing him from following in Ol' Obi-Wan's footsteps, why would this seem to just slip his mind. Perhaps it was Threepio's gold finish, though faded. I can't seem to come up with any conclusive answers to this little pondering of mine. Please let me know all your thoughts. May the Force be with yall!