What might these inscriptions added to an «Easy Money» board mean?
The P looks like the payoff value for the mortgage: It's the M value plus 10% (mostly -- it looks like some of the numbers are just approximations to this value or round in arbitrary ways: 175 + 10% is rounded down to 192, while 395 + 10% is rounded up to 435). So: a remedy to having to calculate mortgage payoffs on the fly (and perhaps to avoid arguments over rounding).
The S value is similar: it looks like the B value minus 10%. Perhaps this was a house rule that allowed a player strapped for cash to sell some houses instead of mortgaging the property: selling a house would thus incur a 10% loss.