Not every grave is the grave of Hiram. On the Chevalier d’Orient (‘Knight of the Sun’) tracing board, there is a “little rectangle with a triangle where in is Jojakim’s grave”. The text comes from the Baylot manuscript, the image from the Kloss collection.
The L stands for Linanon, the mountain where the wood came from in this degree.
Kloss Collection de 84 tableaux (1784) Kl.MS:XXV.1