Historical clue formulations

• Unknown quantity in a 10-Down — 2023-09-14
• The "x" or "y" in 2x + 3y = 15 — 2019-10-08
• Not fixed — 2019-09-07
• In the answers to 17-, 34- and 57-Across, it was replaced in turn by 1, 2, 3, 4, 5 ... — 2011-08-11
• Like some manufacturing costs — 2000-08-16
• Something not to be relied on — 1995-10-14