Seems right to me

Bit cheeky, just nicked your own words. But why make it more complicated. That's what they were - call 'em that!

prbalby in some kind of GUILD or UNION...also....

I guess military terminology is in order since they worked for the army
"A partir d'avril 1939, les volontaires formeront des COMPAGNIES DE TRAVAILLEURS pour le compte de l'armée française.

En voici deux exemples :
151e COMPAGNIE DE TRAVAILLEURS ESPAGNOLS, à SALBRIS (Loir-et-Cher),
184e COMPAGNIE DE TRAVAILLEURS ESPAGNOLS, à BALLOIRE (Maine-et-Loire)."
http://www.apra.asso.fr/Camps/Refugies-Espagnols.html

"These Spaniards came from labour companies which had been organised in 1939 and 1940 in France, or were delivered by the Vichy Government to the Germans."
http://www.nizkor.org/hweb/imt/tgmwc/tgmwc-05/tgmwc-05-43-06...

Type in "compagnies des travailleurs étrangers" at Google and you have just three hits. The first snippet is the most interesting but is not accessible (being a paid on-line journal) and fits in with the "internment" bit too. "The result was the creation of the first colonies of foreign workers (Compagnies des travailleurs étrangers) with about 55,000 former participants..." It might be far-fetched, though not completely implausible, to think "compagnie" as a military "company" as "brigade" (though it would be "brigade" in French) is mentioned in the abstract below.

"After Hitler came to power, France had been one of the most important host countries for emigrants from Germany. In spring 1939 another wave of refugees reached the country after the Spanish Civil War, whereupon the government of Édouard Daladier reacted by building internment camps in the south of France. The Third Republic used this huge potential of human labour and resources for its own economy by creating special colonies and brigades of foreign workers. As a result of external tensions, but also on the basis of the great number of internees, the government prepared to integrate the immigrants in additional foreign units in the French army, thus establishing a common aim of fighting the nazi dictatorship. The German-Soviet Non-Aggression Pact of August 1939 and the outbreak of the second world war caused the French Home Office to do an about-turn. In an atmosphere of anti-communist hysteria, antisemitism and xenophobia, Daladier articulated his mistrust of communists and 'hostile foreigners' and as well as arrests, ordered the mass internment of immigrants originally from the territories of 'Greater Germany'. With such repressive measures the Third Republic robbed itself of the opportunity to use the political ambition of declared enemies of nazism in the fight against persecution and tyranny."
(http://www.ingentaconnect.com/content/sage/jch/2000/00000035...

Prestataire is a person receiving benefits or allowances. They may have been receiving an allowance for work in prison.
Not sure how they could receive an allowance from Spain as was during the war.

or:

Spanish workforce consulting companies

Prestataire means: entreprise qui vend des services, or, consulting company. In this particular case, the workers, the labor, comprises Spaniards.