My First Box Of Halves

Can't tell what mint mark that 1976 has looks like a D and the 99P makes them both very shiny but not proofs.

The Royal Australian Mint has been using nitride hardened or chromium plated dies for some years now. That has the effect of producing proof like images for business strike coins, although they are NOT proofs.
The coins themselves still fall on top of each other, onto a pile out of the minting presses, to sustain 'bag marks' as they have always done.

The difference is to discern if those marks were sustained as a result of the final act of minting, or were acquired in circulation.
Either way, as with those coins pictured here, the motivation is to save the best condition coins, of all types, varieties and mint marks.

With the American halves, there is the added attraction of striking silver. However, if trying to find silver were my only is your motivation, I would feel that there would not enough silver out there now to maintain my interest. My motivation would have to be for numismatic interest, not bullion reasons.

Just keep trying Mikem, you are correct on the assumption that your coins have been previously searched. Unless you dump them into a different district, you will eventually get those same coins back again, with a possibility of a little silver mixed in.