In short "given past form" means in one word historically, That is looking back these shares have consistently performed well in producing a relatively good dividend ratio.
Most dictionaries do not explain this well in https://en.wikipedia.org/wiki/Value-form you will see the term "form of value"
It is not simply that "money becomes the general expression of the form of value of goods being traded" there is a secondary meaning of "given past per-form-ance" similar to that used in "on form" similar to the way we use it for sport "His form this season has been brilliant"