Woah damn. I always thought 3131L was a fleet add. Cause when it
came in, IIRC 8205A was still stuck on Sv 12, though like you said
some permers swapped timings. Plus 8205A went to Sv 39 only after
the timetable revision for Sv 39, which is about a month+ after
3131L came in.
So the period in between after 3131L came in, I don't think
8205A did much cameoing though other than on Sv 12.
Weird. 
Just to check again, the fleet add for 538's 2nd trip, the fleet
add is one of the 2 Batch 1s? (9345Y / 9317D)?
8205 was running around bedok and tampines route group services
while spare. i go to the east less often now but i still see that
bus running around on routes that go to where i go.
yes the fleet add was when 9317 9345 came in... that one dates
back many months ago. also round the time 7433 replaced some other
non-WAB bus.