MK gearbox selectors

[tck]

I have two MK gearboxes one in the 5000 number range and one in the 7000 range  so they cover  MK4-5 I am trying to sort out which box is in the best condition the 5000 looks OK apart from the expected worn selector pins and nut 26396 that secures the pinion was missing !
The other 7000 box looks OK too, the major difference being that the 4000 box has a short spindle for the selector quadrant terminating in a pb bushing in the internal pillar casting while the 7000 spindle is as per plate 17 with a spring. there may be other differences I have not yet seen yet
I am wondering
a. which is the best design? (I note that often with the MK machines latest is not always best as the companies economics intruded on best practice)
b. how interchangeable are the components?

[tck]

typing error its a 5000 not a 4000 box

[tck]

On closer examination and measurement the two the two selector shafts are the same length but the 5000 series box does not have a spring 4090 ( 17/7)  is this another omission of the assembler or a design change?

[tck]

Picture added below  note No 5000 series is the one with spring

[eddie]

The 5000 series gearbox (with the spring) is the standard gearbox - it was available with a short quadrant shaft for the T35 and Mk3, and a longer shaft with gear indicator for the Mk4, Mk5 and Dragonfly.
The other gearbox (without a spring) had the long shaft and was used on the Mk3 Sports.

   Regards,
                  Eddie.

[tck]

Thanks Eddie I shall stick with the Mk4 plus the missing nut!
many thanks for helping me with this mongrel engine I have the main engine unit to come which i approach with some tripidation