In 1971, DC changed the format of their standard comics from 32 pages for 15¢ to 48 pages for 25¢. This conflicted with each of DC's existing giant series (Series "G", DC Special, Super DC Giant, and 25¢ Series) which were already 25¢ for 64 pages.
Rather than raise the price of their 64 page giants, DC pioneered a new format the 100-Page Spectacular, 96 squarebound pages + covers for 50¢. (Note: The last 4 issues of Series "G" were 64 pages for 35¢ since they were stuck between formats during the transition.)
Like Super DC Giant, the numbering system of the new 100-Page Spectaculars was confounding. The series started with #4 (Weird Mystery Tales). Issues #1-3 do not exist. Perhaps as Jon McClure hypothesized in Comic Book Marketplace #69, #4 actually stood for Vol. 4.
After three issues of DC 100-Page Super Spectacular, DC moved Giant Series "G" into the format. They added the letters "DC" to the numbering system and tied each issue to regular titles. Superman #245 became #DC-7. The pattern for content of each release followed that established by Series "G".
After just six more issues, DC temporarily suspended the 100-Pagers with #DC-13 (aka Superman #252) cover dated June 1972. The series was revived a few months later in February 1973. However, the numbering and content were no longer tied to ongoing titles.
Again the new system lasted just a few months until #DC-22 November 1973. An advertisement for the next issue DC-23 featuring Shazam appeared. When the issue was released, it was again tied to the numbering system of the regular Shazam title (#8). The number DC-23 does not appear anywhere on the book, nor would any subsequent 100-pager contain the "DC" numbering system.
This last numbering change proved to last the longest. In 1974, DC expanded the number of 100-Page Giants in their own titles. Several titles were one-shots, while others were regularly in the 100-Page format. DC also soon raised the price to 60¢. The format lasted until 1975 when inflation once again caused DC to shrink the page counts of the giants.