Hi,
I believe JP has covered the whole topic in single go, a few additions will be the O will be indicator for older periods and if there are more than one retro the WT /553 will be updated.
You cannot see the retro results as such but the differences occurred due to retro changes in MD can be seen which reflect in the WTs as mentioned by JP.
While running the payroll please check the DT (Difference table) processed by PDT function, these calculations can be seen in T.code PC00_M99_CLSTR. Also please go through the URR0 schema and X042, 43 PCRs for more insight.
BR
Upen