OK got your attention now.
What are the little rectangular flaps (usually about letter size) on the back of some reefer trailers for?
ADR markers I think.
do you mean the peep holes 
dave:
do you mean the peep holes
That’s what I think he means.
I’m presuming they are there to ‘vent’ the trailer between loads.
Certainly if you’ve been carrying garlic or onions, and your next load is going to be butter or fresh cream, then the odours need to be dispersed in some manner.
Yes it’s a small hatch. It’s just it seems so small to be of any real benefit.
Yeah they’re for airing the trailer.
Talking of reefer trailers, which does the green square with a black ‘d’ in lower case in the middle of it sygnify? A lot of reefers have got them on now. 
It’s usually stuck on the rear doors somewhere and the plate is about the same size as an orange ADR plate.
By their very nature, Reefers need to have very efficient rubber seals around the doors. This sometimes makes them difficult to open or close. (A ‘vacuum’ effect.)
Hence the vent flaps to relieve the pressure.
Similar vent flaps are also found on the drop down moving internal bulkhead inside a split compartment reefer body.
Sometimes, through lack of maintainance usually, the top rubber channels of the seals can retain water and freeze up which then makes the door bloomin’ difficult to open anyway !! 
i’ve never used them to make opening the doors easier.
we used them instead of the fridge when carrying onions, open the front vents and the rear ones to stop the onions sweating.
the green squares with letters in are for transiting italy thru the weekend and holiday bans
Deesider:
:idea:
By their very nature, Reefers need to have very efficient rubber seals around the doors. This sometimes makes them difficult to open or close. (A ‘vacuum’ effect.)
Hence the vent flaps to relieve the pressure.
Similar vent flaps are also found on the drop down moving internal bulkhead inside a split compartment reefer body.Sometimes, through lack of maintainance usually, the top rubber channels of the seals can retain water and freeze up which then makes the door bloomin’ difficult to open anyway !!
Is that true, or have you just made it up ?!! Source? I like the the theory to be honest with you.
Nice to see you back on here, Rob, as argumentative as ever…
Not sure which bit of that post you think I am making up, so:
Vacuum effect - I read it in a manufacturers sales leaflet about 10,maybe 12 years ago, a French one, Lambaret or Chereau, I don’t remember which, (but I should add it was translated into English!) 
Vent flaps fitted on Internal bulkheads - saw this with my own eyes on the trailer I had yesterday…you’ll just have to believe me with this one. And on a different trailer last week, and a different one again before that…you get the picture.
Back doors Freezing Shut - again, just my personal experience, on more than one occasion. Maybe it has happened to other TN members as well who could confirm it …
I’ve had doors freeze shut, but never had a reefer with a flap.
Volkswagen Beetles were the same, you had to open a window to shut the doors.
What is special about that special spray used to lube fridge seals?