Your original number GB3956 was from a 1956 NH350 - presumably someone overstamped it when they converted it for SQ4 use.
The different mainshafts can be identified from the length between the 3rd gear splines and the clutch-centre splines. The photo above shows a mainshaft from a later NH gearbox (3630-56) where this length is 3-1/64". The length for a SQ4 mainshaft (3629-52) should be 3-1/2".
I ordered one ,when it came the diameter is to wide...
To which diameter are you referring? As far as I am aware the shaft diameters are all the same.