CH 1924 167AII 50 years UPU - yellow gum - sheet - unused

Switzerland 1924 50 years Universal Postal Union (UPU),
Old town hall in Bern, 20 Rp. red with yellow gum, Box teeth,
original sheet (folded) with 25 stamps - unused, most of time mint MNH, stamp lower right without gum (see pictures),
Catalog SBK 167 AII, Michel 192Ay.