Picked up a card off of Ebay a few weeks back that it the new "oldest card in my collection" (at least oldest card that depicts a particular player). This is a card of Kazuto Tsuruoka from the 1947 Marutoku bromide set (JBR 73). According to Engel, the cards in this set show the players in their uniforms from 1946 but it is considered to have been released in early 1947. It is the earliest checklisted post-war baseball card set. (I'm assuming that means Gary Engel knows of other 1946 sets but doesn't know of enough cards yet to make a checklist, but I could be wrong.)
There's a couple things I find interesting about this card. I'm pretty sure that the top line of text says "Hawks" but the team was known as the Kinki Great Ring in 1946, hence the "Great Ring" lettering on Tsuruoka's uniform. The bottom line of text says that the player's name is "Yamamoto" - for reasons that I've never seen explained, Tsuruoka went by the surname of Yamamoto until the late 1950's.