Since the establishment of the PRC in 1949, Mainland China made a lot of changes in 1952, 1958 and 1983 to the original code assignments due to the Chinese character simplification and the need for new characters and symbols.
See CN1980-supreassigned.jpg (Although these were reassigned in 1952, this table is not in the 1952 code book.)
China added some characters to the empty cells in the main body and 7903–7927. For example, 0058 was originally empty, but China added 苯 there.
See CN1980-added.jpg (According to 怎样使用《标准电码本（修订本）》, these were added in 1958.)
China not only officially adopted simplified characters, but also
For example, 穀 (4474) was deleted because it is merged into 谷 (6253) in simplified characters;
彿 (1765) and 髴 (7572) were deleted because they are variants of 佛 (0154);
See CN1983howtouse-variants.jpg (There are 28 pages in total; only the first and last pages are scanned.)
and other uncommon characters (1681 characters; not explicitly listed) were deleted.
Brief history of China's telegraph code (in Chinese) at the beginning of 怎样使用《标准电码本（修订本）》
See the original pages in pictures, or in Chinese text.