unmounted mint 10c Lindbergh booklet pane (C10a), the rare position 'O' showing guidelines at left and top, a unique collectable position on the sheet, made that much rarer because the sheet was normally cut below the top guideline (thus removing it from most 'O' position panes) so as to avoid cutting into the bottom stamp of the booklet pane directly above (position'L'), this significantly reduced the number of identifiable 'O' positions available to collectors, exceptionally well-centered for this booklet pane, never hinged,