Like the Emperor’s new clothes, the answer can only be revealed to those of great intelligence. 
Ok enough silly answers. The 5 blade fan was certainly an aesthetic choice, and possibly influenced by all the aircraft parts work the factory had lately done, but the 6 blade fan was certainly cheaper, and easier to balance, probably moves more air for better cooling, the spider was already in use on the Mark V with longer blades, and there is also the noise consideration, the staggered blade angles helping to reduce a phenomenon called “beating” or a rumbling sound, a characteristic of some helicopters such as the Huey.
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Yes, according to the parts book, the 5 blade fan was used on all early 120 engines up to W5464, so that would be about mid '52.
Also on early Mark VIIs.
So that’s 4464 XK120s and 1000 Mark VIIs that had it.
Thinking about it, I suspect the decision to eliminate the 5 blade fan may have been related to the Mark VII and changes in the water pump and a desire to standardize the water pump across both models.
All this is speculation after the fact, about the best we can do now.
Well, here are a couple more pictures of this old ‘140 fan.
The first pic shows the front side of the fan.
My lack of great intelligence (easily demonstrable !) had prevented me, for a long time, from noticing the reversed coloring of these particular fan blades.
Here is a pic of the rear side of the fan
And a close up of the rear side of one blade
One possible explanation would be: this fan started out being painted in the conventional manner, having unpainted fan tips with black painted spider and inner portions of the blades. Close inspection shows small portions of original black paint still on the inner portions of the fronts of some of the blades.
This engine was removed from vehicle in the 1960s and has sat inside various garages ever since.
Over time, the unpainted blade tips, exposed to the atmosphere, became corroded/tarnished/stained, such that they turned very dark almost blackish in colour.
Meanwhile, most of the black paint fell off the inner portions of the blades, especially on the front side.
This fan appears the have reversed its own colouring scheme , by means of corroding and having some of it’s paint falling off ?
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Thank you Gary, more evidence of interest to at least one person on this forum. 
The question came up when I was writing the Mark V judging guide, hence my unusual interest.
Another possible reason for the change in paint scheme was so that the rivet heads could be painted easily with no extra effort. But as they say, speculation is free.
I should mention that the factory went to an 8 blade fan with the 140 and latest Mark VII, apparently abandoning the advantage of staggered blade angles in favor of another advantage of increased blade quantity.
Maybe Sir Bill had a cousin in the fan cottage industry?
Are you going to have the pump rebuilt? I have one just like it but I have the banjo’s. Mine needs a rebuild and I am looking for a reliable reasonable rebuilder Ed
No plans to have it rebuilt, Ed. That may change once I fire the engine up, in which case I’ll be looking for a reasonable rebuilder too.
There you go, Rob
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Ooooo, yer gettin’ close!!!
Yep. The chrome shop shipped my rechromed parts out this morning so they’ll arrive sometime next week. I’m working on jobs best done with the body off to minimise work. This weekend going over the brakes - I rebuilt them back in the 90s so need to assess whether they need new kits. The m/c features these weird but especially important tilt valves. Not going to trust them after sitting so long.
Just a note. On my FHC 120 the flat bar which spans the front of the chassis/body to which the bumpers fit was not installed when the body shop put the body onto the chassis. Trying to install it with the body on is a freaking nightmare. There is mention of this in the archives.
Best regards
Stay safe
Thanks for the reminder, Phil. I actually think it’s impossible to fit that spring bar after the body’s back on the frame. The steering column is another one best slung in the body as it’s lowered onto the frame.