Sitting Helvetia Stamps With A Fiber Appearing Network On Back

I have two Sitting Helvetia stamps denominations 5 (tan/brown) and 25 (green) with network of fibers on the rear. What type of paper is this and when where these made and are they rare, what value do they have? There also appears to be an oval stamped area on the back what would that signify?

These were printed on, inter alia, granite paper, which is characterised by fibres within the paper. The shape you see is an impressed watermark.

The granite paper Sitting Helvetias had a short run starting in 1881, before being replaced by the Standing Helvetia series.
1881 Sitting Helvetias are a prime example of a postmarked stamp being worth more than a mint version, in particular the grey 40c.

What made this set more valuable as used than mint? Especially since it's a lower grade of paper?
Maybe there was a meeting of the leading catalogs' managers and one said "Let's play a joke on the collectors, and decide to evaluate the 3rd series of the sitting Helvetia as used worth 10 times a mint". And the others said on to him "the collectors are not that stupid, they'll never fall for it" and the first one replied "wanna bet?"

Maybe you should spend less time on thinking up all kinds of conspiracies.

Another 'prime' example of cancelled stamps being worth more than mint are the German inflation stamps.
Stamps with short print runs, and especially stamps that were in use for a very short period had very little use. Their relative rarity of available stamps compared to asked stamps by collectors drives up the price.

Ireland had a reprinted overprint just before the new 'free state' overprint was released. The mint set is expensive. The used set is even more expensive because it was hardly sold before being replaced.

Catalogue prices for used are for very fine examples. For used, that includes a clear cancellation for the period of use.

So it's what RuckusCollector said, "The granite paper Sitting Helvetias had a short run starting in 1881, before being replaced".