How does plus/minus square income work?

In the game are two types of squares that directly modify your money. Plus "+" squares, or blue-coloured squares in more recent installments, and minus "-" squares, red-coloured later. When a player lands on such a square, a slot wheel full of amounts is spun, lands on one. and you receive/spend said amount, pretty simple so far.

But the amounts are not consistent. Some things appear to affect them, most notably the current month. What are the exact mechanics?

In particular:

1. How many different ‘slots’ are there?
2. Is the probability even or weighted? (E.g. p=1d12 vs. p=2d6 in rpg terminology)
3. How exactly does the month affect the price?
4. Is there a formula, or would a lookup table make more sense?
5. Does the state of the board (the year, or how much property is owned) raise the amounts in the late-game, or is it time-independent/static?

So far it appears to be of the form a + pb, with a and b dependent on month and type; plus or minus. I don’t know the values of a and b.

Background info

The currently very popular digital board game Momotarou Dentetsu is something Japan-only, and a long-running series (since 1988). There’s preciously little information to be found on the internet. The games are remarkably static: much like Pokémon, game mechanics change very little from installment to installment. Usually a few things are added with each new game, with there being 23 games total so far (with most English gaming sites only listing the first two).