Interesting this is your second post of items similar to items recently posted on the LastDodo stamp forum.
https://www.lastdodo.nl/nl/communit...e-postzegelsYou will need a specialised Edifil catalogue volume that lists the so-called Emisiones Locales Particulares (ELP numbers), or the catalogue by Julio Allepuz Querol (also published by Edifil). These are from the nationalist territories.
Only very few nationalist stamps issued were issued with the consent of the nationalist government. The only overprints on nationalist issues were those paying the fees for carriage to Lisbon - Burgos - Germany from Las Palmas, on board Lufthansa's flight TO-191.
Many private persons bought stamps and had them overprinted so they could sell them for a big profit. As the nationalist government had little reason to invalidate stamps that had such overprints, it, not necessarily, invalidated them.
That said, generally, they are bogus. Some were made at a later date. As more research is done, more approvals by "local" commanders are found to be bogus or antedated. Edifil is heavily criticised for listing such stamps.
That two very similar sets pop up within about a week is very odd. Most private overprints were applied on more than a single stamp from a set, usually "Cifras, Cid e Isabel." Both your set and the one posted on LastDodo focus on the "Cid" 5 cms brown. To me, that would raise a big red flag.
If they are listed in the catalogues I mentioned, there is a market for them. However, that does not change that they are dubious or outright bogus.