"The lintels are undoubtedly early. Continuing studies of the style of carving on Yaxchilan monuments show that attempts to reproduce styles from an earlier era by Bird Jaguar IV's artists resulted in a "handwriting"recognizably different from the style in which all Structure 12 lintels were executed. However, architecturally, Structure 12 does not seem contemporary to the reign of Ruler 10. Other structures at Yaxchilan known to predate the Late Classic (such as Structures 6 and 41) are constructed with much more massive masonry than Structure 12."
Carolyn E. Tate, Yaxchilan: The Design of a Maya Ceremonial City, p. 170
Note the presence of the Tikal emblem glyph in the extreme lower left of the inscription.