Old part number 42600a3 new part number 42600a09 what is the extra white wire for on these ??

I wish I could help you - I checked in my repair manual - however this is what it said - there is no mention of a white wire - We have a 2000 7.4 mpi all stock

a - ORANGE Wire - From Electrode On Transom Assembly

b - RED/PURPLE Wire - Connect (Other End) to Positive

(+) Battery Terminal

c - BLACK Wire - From Engine Harness

d - BROWN Wire - From Electrode on Transom Assembly

3pointstar

Possibly the white wire found on the black box (vs the old blue control box) is a connection point for the latest type of Merc's MerCathode corrosion monitoring system that on some models seem to be using a green flashing light to help let the user know if the system is working correctly or not as indicated in the following video from Mercruiser,
Hum Well I cannot seem to be able to make the You Tube video work above via a link so you'll have to search online ( MerCruiser 4.5L / 6.2L Sterndrive - MerCathode System ) and watch the video if the following two tries don't show up either.... https://www.bing.com/search?q=MerCru...R&pc=EUPP_DCJB

https://www.bing.com/videos/search?q...4EEA&FORM=VIRE

Your vid link is above.

For some stupid freakin reason most links don't show up on this site until they are quoted.

Does the new one cast a more powerful field than the old. Most posts I've seen wound up with the owner requireding 2 per engine to make a field powerful enough to prevent corrosion. Testing for field is a PITA too.