I’m afraid I don’t have a BB54 to confirm, but remember reading something on WUS about the difference in springbar width between rubber and bracelet options with the 54. I’m certain that you’re right about the rubber needing 1.5mm and the (BB54, at least) bracelet using 1.8mm.
On a bit of an aside from your situation (although it might affect you as well?), but apparently the endlinks on the rubber to fit the BB54 are also different to those on the bracelet. A chap on WUS tried to add the rubber whilst using the bracelet endlinks, but the smaller springbars meant they didn’t stay in place and started to rotate outwards when pressure was applied to them.