I queried my friend Walter about this one, he's an English scholar, and a lefty from way back -- his father a union organizer in the 1930s. Here's what he emailed to me:
It doesn't seem like a very odd phrase to me, considering that "meal ticket" is very similar in meaning, and common, but also not entirely self-explanatory. That is, "a ticket for a meal" is clear enough at its immediate level of meaning, but you need a context for its connotations. Same with "a card for pie." Anyway, here's OED stuff. First group are simple definitions, second group are historical examples of usage, all of course from printed sources..
pie-card U.S. slang,
(a) [1] a meal ticket; [2] one who begs for a meal;
(b) [1] a union-card; [2] the holder of a union-card; also attrib. [my brackets for clarity - wl]
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1909 W. G. Davenport Butte & Montana beneath X-Ray 56 Say, on the dead level, Andy, couldn’t you let me have a lonely ten spot?.. Say, Andy, you’ve just got to jar loose with five bucks for my *pie card is so full of holes it looks like a piece of mosquito bar.
1929 Amer. Speech IV. 343 Pie card, a union card used to obtain food or lodging.
1931 ‘D. Stiff’ Milk & Honey Route 211 Pie card, one who hangs around and lives on a remittance man or some other person with money.
1945 Seafarers’ Log 25 May 2/1 The Commie stooges and pie-cards kick us around.
1948 Mencken Amer. Lang. Suppl. II. 678 Pie-card, a union card used as a credential in begging.
1960 New Left Rev. 26 Sept. 41/1 The retired..comfortably fixed pie-card artists of every lost..cause of the labour and radical movements.
1973 C. Rubin Log 64 All of them phony, pie-card officials who sit on their big fat asses and twiddle their thumbs.
