Sid Check was a real artist and not a pseudonym. He did indeed do plenty of work for EC Comics, and his wikipedia entry notes that "Sid Check was often compared to Wally Wood because of a stylistic resemblance". Check was a friend of Frank Frazetta's and also collaborated with him on occasion. When Wood worked together with Harry Harrison (and Joe Orlando) in the same studio, Sid Check was also a member of that same crew. Before Bill Gaines locked up a lot of his artists as exclusive to EC, Avon employed many of the same talents that would later go on to fame working for Gaines. In this issue, Everett Raymond Kinstler did the inside front cover, and Jack Kamen did the last story. "Case of the Emerald Robbery".
Link to the book:
Gangsters and Gunmolls 2