PC Engine controller PCB "PWD-624" - variations in d-pad contacts?

[micro]

Recently I've obtained a PC Engine (the standard white one) with two controllers. Both controllers are the PCE white ones, one with turbo switches, the other one without.

Inside both controllers is the PWD-624 controller PCB. (One PCB is lacking the extra IC for turbo fire.)

What really did surprise me was the fact that on these two PCB's NEC used the same circle-shaped contacts for the d-pad like they used for the other buttons: (bottom PCB)



I've got an extra controller PCB (top PCB) that got a different type of d-pad contacts, resembling more the ones used in NES and SNES controllers and shifted by 90° for UD/LR. That's also the type of contact that I've seen on every other PCE controller PCB pic on the internet. 

It seems NEC at some point ditched the circle shaped contacts for the d-pad and used the other type from now on, doesn't it?

This seems also backed by the date codes on the IC's used on the PCB's shown on the pic above.

bottom PCB:
74HC157 - 10th week of 1988
74HC163 - 14th week of 1988

top PCB:
74HC157 - 52nd week of 1990
74HC163 - 16th week of 1991

Is this a known fact? Are the d-pads of early versions of the PCE Pad known for being inferior?
Somehow I couldn't dig up any informations on the different types of d-pad contacts. Lawrence didn't even mention it in his elaborate PC Pad review (http://nfgworld.com/mb/thread/460-PC-Engine-TurboGrafx-16-Pad). 

[NFG]

I think you'll find there's no functional difference.  My guess is they changed them to promote longevity.  The solid pads have more of the carbon contact area, and will stand up to more abuse.  It can be a problem if the player really grinds the contacts hard when playing, but finding a PCE pad that's so worn it fails to work properly is incredibly rare.  Probably they made the change and it never proved necessary.

[micro]

I don't know, man... With my wireless controller PCB's I've found out the hard way that the shape of the d-pad contacts must match the characteristics of the controller and d-pad.

Used easy-to-trigger contacts on the wireless SNES controller PCB -> the diagonals would trigger to easy, even if you just wanted to press the 4 main directions.

Using the d-pad contact style with the cut in the middle of the contact on the wireless Saturn controller PCB proved equally devastating: Now the diagonal directions would be very difficult to trigger...

Back to topic: I also found out that the thickness of the d-pad disc used in the PCE controllers differ:

http://imgur.com/LqtmoPr

The right one is about 0.5 mm thicker. Unfortunately the parts inside my PCE pads are "mix and match". But it seems the thicker d-pad disc was used in the later PCE pad versions with the rectangular d-pad contacts.

Using the thin d-pad with the rectangular d-pad contacts makes the diagonal directions VERY hard to trigger. Rectangular contacts + thick d-pad is ok, but still the diagonals are easiest to trigger with the circle-shaped d-pad contacts (regardless of the d-pad thickness).

At least that's my conclusion, however I'm definitely not a PCE expert. 

[NFG]

Yeah, you're not wrong about the design having different requirements for the rest of the pad mechanism.  It makes sense - the rectangular setup, with a single split, creates a very narow active switch zone.  The circular system creates a much larger area where contact will have an effect.

This seems backed up by the idea that the different d-pad discs respond differently based on the PCB used. 

I wonder if we'll ever know why this change was made.  =/