If I am not mistaken, thats the back of the same form we made them give us at the stung treng border.. He wasnt going to give anything, and was angling for soem money so we were also hesitant to make a problem, but did in the end.
If its the one we got, its more a customs declaration for bringing valuables in, like an 'anything to declare' form you would associate with carrying in valuables.. Was all he had in his desk so just used that.
Ours didnt get a vehicle stamp like that.. just some scrible by him, the 'declared value' of the bikes and a signature. No one asked for it (even in sihanouk) and it wasnt given in on exit. There appeared no end date or anything firm. As we could have come in without any of this if we didnt literally wake him up sleeping in the shade, and push him to give one, I doubt theres any working time limits in reality.