This NOS Original Equipment Crown Wheel and Pinion set was Manufactured on November 20th 1963 and I cannot find any model listing for the Part No printed on the box being 600355 but the box is also marked with the Part No 608109 which is a Supersession of the first Rover 2000 P6 Part No I can find being 556432 .  These later Part Numbers were fitted to 2000's up to Chassis No 40277337D.  The fact that Rover P6 commenced Production earlier in 1963 but was not launched until October fits with this information and was probably the original part no which does not show in the later supersession lists..