Flaws
At that age they don't have ISO or CAD. Every ship in same "class" will have slight difference. Some because different manufacturers. Some because improvement from previous ship(different batch). Some because special design (like the No.1 ship is designed as fleet ship so it will has extra room for commanding). Ships in same class might have 1 or even 2 knots speed difference in test.
But as a game, please don't against players. Flaws can be a very annoying feature. My suggestion is make it optional. And provide enough methods to overcome it(tech, extra cost, extra construction time etc.).