I am also thinking that this could be P. glabrum.
Please check the following;
Nut biconvex- P glabrum
Nut trigonous P. barbatum
But, according to gamble, there might be biconvex nuts among
trigonous nuts so check for the presence trigonous nuts in the
Bracts not ciliate on the margins . P. glabrum.
Bracts ciliate on the margins - P barbatulum
Ocreae (A sheath formed at the node of a stem by the fusion of two
stipules) 2-4cm long, tubular, glabrous, truncate at mouth - P glabrum
Ocreae 1.5-2.5 cm, tubular, strigose (Having stiff, straight, closely
appressed hair), with very long ciliae at the mouth- P barbatum
Hope this key would be helpful.