I agree it has not been a mandatory requirement in the past and I have signed many PoLs. Previously on the PoL it said "place your stamp in the box if you have one" (my words) but this year, by signing the form, the witness certifies that they HAVE put their stamp in the box. I had gone to my mates house to sign his PoL but when I discussed it with him, he agreed that there was an element of risk (of the form being sent back) if I signed without a stamp. I said I would sign the form but I would either cross out the the bit saying I had put my stamp in the box or write on "I don't have a stamp". I'm not telling a lie on an official Govt form. We both ended up going to our local Doctor who did it for 200 baht.