The kanji 年 is a generic "unit" for year, just as 月 is for month and 日 is for day of the month. 年 is used regardless of the era or the calendar type. The calendar type or the era name can be specified before the number:
- 紀元前3000年 = 3000 B.C.
- 西暦2023年 = 2023 A.D.
- 昭和48年 = Showa 48
- 令和5年 = Reiwa 5
- 宇宙世紀78年 = Universal Century 0078
In your case, it is a convention to use the Japanese era system in these vehicle inspection stickers, and the era name (令和) is not explicitly specified. In other words, you have to remember that it's in Reiwa. This system works because we understand these stickers are valid only for a few years. In other business fields, such a convention is clearly confusing, so people just use the Western calendar or add the era name explicitly.