Good question Numisma! I find that 42 issue early sub-set more interesting for several reasons. Based on mintages, around 3 out of 4
Mercury dimes were minted 1934 and later; the early set (pre-1934) is currently more valuable, and I see that value increasing sooner than the late dates. Aside from that, those early dates likely circulated more, so finding choice specimens is going to be more of a challenge. Look at any price guide and see how dynamic the valuation on those early dates is compared to the relative static valuation on the late dates. It is a vintage set that seems far more interesting with respect to collectibility. I'd love to hear what others think about collecting early mercs.