This is one of the subsidiary clamps at Ballybrophy - they are all fitted with the nuts to the outside of the track, which seems to be standard practice.
There are two general heights for double deckers - 14'6" AND 13'6" - I would imagine that the UTA didn't have many low ones.
Thus 58 or 54mm in 4mm/ft.
On two out of the eight 141/181s that I've been associated with, we needed to shave a tiny bit off to ensure clearance when the plate was fitted, the other six were OK*.
* or might just not have had the plates fitted...