Mention must be made here regarding the cleaning of the keyboard.
The unit above it particularly dirty - with some corrosion.
The original instructions suggested 'occasional use of a light abrasive metal cleaner' to maintain proper performance. The obvious implication is that the board could wear through with over cleaning with abrasive cleaning materials. It is, after all, only printed circuit material

This example has been removed from the case prior to a good cleaning of the keyboard.
The white lead is to the stylus; the red and black leads at left connect to the battery.
This is a standard white model with plain keyboar d.

In this case, chrome cleaner paste and a lot of elbow grease is called for.
This particular board came up very well indeed after its' cleaning, and despite reservations about being too harsh on the board, this may well be your only option to get it back to something like a respectable finish.
After cleaning, there were no signs of wear at all, and I would suggest this method without too much worry. one area to be careful of however, are the narrow 'tram lines' surrounding the keyboard; break through one of these and your into a lot of unnecessary, time consuming work!.

A portion of the same keyboard after cleaning.
The board looks as good as new, and with care to the surrounding areas, no damage has been done.

The preceding pictures are basically to show you what's inside your Stylophone, and unless your own model is showing this sort of dirt or corrosion, it would be best to leave it as it is.
Some models have boards that are easier to remove than others, and some flexing of the board may occur.
This could lead to a cracked board, or de-soldered joints...nightmare!.

So what are the differences in the circuits?

Well, it's more the components than anything else. As newer, and smaller resistors, diodes, transistors etc. became available, they were incorporated into the board. This makes for fascinating investigation when examining several units, as you can see the progress through the years as the more compact bits and pieces, and more efficient no doubt, (resulting in longer battery life), are used in manufacture.
Rule of thumb: Big bits - Old Stylo.
Small bits - New(ish) Stylo.

This is not a hard and fast rule, as components could have been used from a number of different sources, and cannot be relied on, but the photos below show some of the obvious differences:

Black and White model with plain keyboard

Wood Effect model with spacer keyboard

White model with black grill and spacer keyboard

Wood effect model with spacer keyboard**

**This model needs a special mention - see Hong Kong page...

Components

The electronics used to drive these little instruments proved to be very reliable, with the only maintanence usually required being a general clean....