I checked both gearboxes I have for stamp marks, and found them easily (on the refurbished box that's fitted to my 2 litre engine) and with the aid of brake cleaner and a rag (on the box originally fitted to the carb 2 litre motor). Both boxes were stamped 14/53 and what I presume to be a serial number was also stamped - 84915 (no leading zeroes) on the refurbed box and 073971 (note leading zero) on the original box. It's tempting to read too much into this, but that adds weight to my suspicion that the refurbished box is a later one as fitted to the i.e. models.
Build date of my Spider has been confirmed as October 1979, and I don't think it was too long after that date that production of the i.e. models started, so I'm hopeful that it's a later box and crossing fingers that it might have the more relaxed 5th gear ratio.
If anyone knows otherwise feel free to disabuse me of these notions...