During operations, a 4" EU insert was mistakenly used in place of the required 4" IU insert, which originated from incorrect parts being ordered several years ago and remaining in inventory without proper identification. Although the Driller performed a fit check, the insert size was not verified against specifications, and no caliper measurement was conducted. The crew relied on the elevator’s “green” latch indicator. Absence of physical measurement as a standard practice, due to the repetitive use of the same equipment across drilling programs. Lack of thorough insert inspection using calipers to confirm correct sizing. Gaps in SOPs and JSAs, which do not explicitly require physical verification of elevator and lifting sub-sizes.