With the second type things seems to be more straightforward. there are 2 classifications:
31 73 4690 1xx-x Sekss indicating that these were used with freight,
31 means
4+ axles, railway-owned65 73 98 70 0xx-x DDm that were used on passenger trains,
65 intdicates
Automobile transport car for passenger trains and DD as mentioned above
for the record
Sekss shows:
S: Special flat wagon with bogies
e: Double-decker wagon for motor vehicles (for the S class only)
k: Maximum load on class C route: m < 40 t
ss: Permitted in trains up to 120 km/h
Although I won't ever learn it by heart like probably Jason does, it's fun to fiddle with it now and then
if you haven't done that before, strat here
http://en.wikipedia.org/wiki/UIC_wagon_numbers and give it a try
concluding, could we have some more info, place and year of built, units, numbering etc
31 73 4690 103-4 Sekss at Agios Ioannis, like all the rest
31 73 4690 115-8 Sekss
65 73 98 70 011-9 DDm
65 73 98 70 017-6 DDm