Sure the Poles took possession of Kar.98a carbines, they are far less commonly encountered than the G98 variation. So far I have not encountered one rebuilt at FBR, but I suspect they were, for whatever reason most reworked by FBR were G98's, at least insofar as I have recorded.
I wouldn't attribute a Kar.98a to Poland by its sling arrangement alone however, that could mean anything. Rifles that made it to Spain, supplied by Germany to Franco's Nationalists or the far left's "Republicans" (what a laugh, more communists than "republican" - anticlerical/atheistic, church and press burning murders...) who were supplied by the USSR (with Spanish gold) typically have other signs we use to attribute Spanish connections. (surplus German rifles were common after both world wars, Belgium and the Dutch peddled plenty after WWI, plenty were smuggled into neighboring countries, no telling where they all ended up..)
Try and do a few pictures of the original (matching) components.
