Peter, it says SEALY BRYERS & WALKER DUBLIN
A quick Google search shows they are a publishing house and no doubt printed the certificates.  I just found a handwritten note on the back of the Optional certificate that says:
Official Promotion Certificates, DM Police and I believe it's in my grandfather's writing.  I assume he noted that so that when he put them away he would know what they were without having to unfold them -- all certificates are folded in thirds but show fold marks from being folded in half at some point.

Thanks for all the info regarding the cost of taking the tests and the signatures on each, more info for my Granddad file!