I've completed an inventory of the pages in the 69 LSE PDFs. That is, for each page in the PDF (or more specifically, for each of the two logical pages on every physical page), I noted whether it was a microfiche title slide, a regular Socialist Standard page, a blank page, or something else. Creating such an inventory was necessary so that I can later split up the pages by issue; I need to know where each issue starts and ends within the PDF.
Below is a chart showing the 13 020 logical pages as they appear in sequence in each PDF. The pages have been colour-coded as follows: white, a blank page; grey, a microfiche title slide; green, a regular Socialist Standard page; red, a "special supplement" that appears to have been distributed with the August or September 1967 issue; and blue, the official Socialist Standard index which was included in some bound volumes.
As can be seen, the length of the Standard varies a great deal, sometimes even within a single month (often due to special anniversary issues, but sometimes for no apparent reason). Indices were included only from 1940–1952 and 1969–1971. Every PDF except for 1915 starts with a title page, and further title pages appear in the middle of the 1922, 1927, and 1933 volumes. The January issues for 1968, 1969, and 1971 are missing their covers; for the first two I will have to contact the Party to get a physical copy to scan myself. Finally, the December 1966 issue appears twice.