Portuguese to English Tech/Engineering Materials (Plastics, Ceramics, etc.) Abbreviation used on mechanical testing certificate

A very unfair acronym to use in a Portuguese-language document!

Before proceeding any further, I have seen the expansion "ordem de serviço" (work order) here: http://www.proz.com/kudoz/portuguese_to_english/mining_miner... it's definitely an option for my question but I'd like to be sure that there aren't any other options that are equally or more likely in the context described here.

The fields at the head of a materials testing certificate issued by the Centro de Caracterização e Desenvolvimento de Materiais in Brazil are as follows:

Cliente:
Contato:
Endereço:
Cidade: CEP:
*OS*:
Data de recebimento da amostra(s):
Periodo de realização:

OS is followed by nine digits in the form xxxx/xxxx-x. It doesn't tie in with anything in the body of the certificate that I can see, and appears to be administrative rather than anything else, given that it appears between the client's address details and the dates on which the samples were received and the tests performed.

I do realise the field is pretty wide, but any thoughts that anyone can offer will be welcome. It is probably not vital so not solving it will probably also be an acceptable option.