What is the test for using the term "my issue" in a will?

In the event the use of "my issue" by the deceased in Article III(c)(3) is not just difficult to interpret but is also an ambiguity then I am satisfied that I may examine extrinsic evidence of the circumstances existing when the will was made in construing the Article. In support I am referred to Boyce v. Cooke (1980) 14 Chancery Division 53 where the court says: "... When it is said that surrounding circumstances may be looked at that only means that the circumstances existing at the time when the testator made his will may be looked at. You may place yourself, so to speak, in his armchair, and consider the circumstances by which he was surrounded when he made his will ... to look at a (document) subsequently executed is not to look at the surrounding circumstances which existed when the will was made."