A colleague asked me to present a Fenestella collection (collected in the Netherlands recently) to this forum which could be referred to Fenestella betulae:

Ascomata developing under the bark (Betula), erumpent, up to 6 together. Ascomata black, globose to ellipsoid with slightly papillate ostioles. Asci about 270 x 18 mu, bitunicate and uniseriate. Ascospores 30-40 x 13-15 um, hyaline turning brownish, firstly one median septum, subsequently 3-septate and becoming golden yellow and muriform. End cells appear to remain paler. Presumed anamorph with hyaline, cylindric conidia with small guttules, about 3 x 1 um.

Little is known about Fenestella betulae described in 1883 by Saccardo. Berlese (1900) treated it as a synonym of F. princeps (= F. fenestrata) but the ascospores of F. betulae appear to much smaller. A modern description does not exist. Jaklitsch & Voglmayr (2020) could not include it in their recent treatment of the genus because of lack of fresh material.
It would be interesting if somebody has more information on this species.

Thanking in advance for help, Eduard