The two viri glyphs on side b are clearly defining 261 (= 9 * 29) days together:
 
258 258
Ab1-1 Ab1-2 Ab4-17 Ab4-18 Ab7-25 Ab7-26
1 2 261 1 260 261
day 1 129 day 131 129 day 261

Ab4-17 is the last glyph (number 261) in the first half and Ab4-18 the first glyph in the second half.