Found the following hamletworks.org entries referring to line 688

View Commentary Note for 684
684 Ham. I will.1.5.2

View Commentary Note for 688
688 Ham. Alas poore Ghost.1.5.4

View Immaterial Textual Note for 688
688 Ham. Alas poore Ghost.1.5.4

View Material Textual Note for 688
688 Ham. Alas poore Ghost.1.5.4