For almost 30 years Bob Kane's little namebox had appeared in every issue as the principal artist on Batman, although it was an open secret at DC that a succession of then-unknown but talented artists like Jerry Robinson, Dick Sprang and Sheldon Moldoff were actually doing most of the work.
But in the late 1960s, things changed. As the Batman TV series collapsed like all fads, the circulation for Batman dropped alarmingly. In Batman #200, DC reported average issue sales of 806,000 for that mag for the prior year. In Batman #210, DC reported average issue sales of 533,000, and by Batman #220, it was all the way down to 356,000.
Apparently sometime around 1968, DC finally bought out Bob Kane for good. At any rate, in Batman #203, a giant issue, Julius Schwartz announced that a new artist would be getting credit for the next issue of Batman:
Moldoff's last original Batman story appears to have been in Batman #199; of course he would appear many times in reprint issues. Chic Stone (another contractor for Kane) did the last few stories, including issue #200.
It took a long time before proper credit started showing up in Batman reprints. In this I don't entirely blame Julius Schwartz, the editor for Batman. It's one thing to know that Kane hadn't drawn all those stories; it's another to affirmatively state that someone else did. I have looked through quite a few reprint books from around then, including Batman #203, #208, #213, #218, #223, #228 and #233-238 (#234-237 featured one Batman reprint per issue in the 25 cent, 52-page format).
The first time an artist other than Kane is given credit in one of the reprint stories is in Batman #233. The opening story, The Death Cheaters of Gotham City, is (correctly) identified as the work of Jim Mooney. But the rest of the stories in that issue are all Moldoff, and none of them are tagged as such; it's just the usual Bob Kane box.
In Batman #234-235, Schwartz reprinted a pair of New Look tales which were obviously Infantino and were properly credited. In #236, he reprinted a 1940s Batman tale called While the City Sleeps, a Dick Sprang tale that is not credited.
Finally, in Batman #238, Dick Sprang's name was mentioned:
Note that the Kane namebox there is pretty blandly filled in (reportedly not by Sprang, who refused the chore); the stuff that Kane submitted as his own but was really done by Moldoff has a much more stylized look as you can see at the top of the post, with the "O" overlapping the two "B"s (not usually that dramatically) and the underlined KANE).
So Sprang gets his first mention in Batman, about 219 issues after he first appeared. However, at this point the crediting was still rudimentary. Batman #239 included a reprint of an early Christmas story that is clearly Jerry Robinson, but there are no credits other than the Kane box. Batman #240 has a New Look Batman story from #164 that is obviously penciled by Moldoff; it is credited to Kane/Giella.
So when did Moldoff get his first credit? I'm still looking for the answer to that one. After Batman #242, the magazine reverted to the normal 36-page format for a year with no reprints. Then from 254-262 the issues were 100-pagers with loads of reprints, but as far as I can see, no stories are credited to Moldoff. After that, reprints moved over to Batman Family; unfortunately my collection peters out around then as well.
Update: See also this post of Bill Jourdain's for the first writing credit to Bill Finger, the scripter of most Batman stories prior to 1964.
Update II: This article on Batmania (the Batman fanzine of the 1960s) contains some tantalizing details:
Jerry [Bails] detailed how Bob Kane had hired the unassuming Finger to write his (Kane's) feature "Rusty and His Pals" and then "Batman." With Batman's success, Bill soon began working directly for DC, co-creating such famous strips as "Green Lantern" and "Wildcat." He was noted for his ability to "adapt the freewheeling style of the pulps to the four-color panels, and break down the action of a Douglas Fairbanks-type adventurer into a panel-by-panel description for the artist."
The K-a article was probably the first anywhere to publicly state that "Bill is the man who first put words in the mouth of the Guardian of Gotham." By Finger's account, Jerry went on, "The cowl and cape, the utility belt and gauntlet, were all Bill's contribution to the dialogue that gave rise to the final form of Batman's famous costume"-along with the Joker and "all the other principals and supporting characters of the early strip: Robin, of course, but also Commissioner Gordon (who appeared in the first Batman story), Alfred, the Penguin, and the Catwoman, as well as the many unusual and sympathetic characters that made the early Batman so popular."
Update III: Jerry Robinson gets a credit in Batman #255:
Update IV: Commenter Lee notes that he read all the Batman books in the 1970s and that Moldoff's name never came up. He suggests that the book, The Greatest Batman Stories ever told (published in 1988) is probably where Shelly got his first credit for the Caped Crusader:
Note the credit at the bottom of the page, just underneath the artwork. I don't know if this is really the first mention of Moldoff, but at least it's a place we can try to walk backwards from.
Update IV: Check out this post of mine to learn where the first Easter Egg with Sprang was located.