One of the hazards of casting is that the rubber moulds deteriorate sometimes at unpredictable rates.
This was brought home to me again this week while I was turning out a small run of Superbarn castings.
The carriage sides are mirror images so they require separate masters and moulds for the clock and engine side (to use FR parlance).
As you can see, however, one side has turned out noticeably longer than the other.
This is because one of the moulds has stretched.
(I did check and, yes, the masters match perfectly, and so too does the smaller side which is still the same length as the original.)
It's possible that it has been through more casting cycles, perhaps as a result of some mis-casts? - or perhaps there was some variation in the ratio of catalyst in the rubber mix so affecting the density of the cured rubber?
Whatever, the only solution was to make a brand new mould, delaying production by at least 24 hours.