There are four legal terms in the sentence, i.e. beneficiaries, designs, heirs and assigns.
The meanings of beneficiaries and heirs are clear. And the sentence used assign to mean assginee. The noun assign is listed in the dictionary:
another term for assignee (sense 1): this agreement shall be binding
upon the parties and their successors and assigns.
Sense 1 of assignee:
chiefly Law: A person to whom a right or liability is legally
Now, I checked Etymology Online Dictionary and to design and to designate have the same etymology as follows:
1540s, from Latin designare "mark out, devise, choose, designate,
appoint," from de- "out" (see de-) + signare "to mark," from signum "a
mark, sign" (see sign (n.)). Originally in English with the meaning
now attached to designate; many modern uses of design are metaphoric
As a verb, from 1791, from designate (adj.)( or else a back-formation
from designation. Related: Designated; designating.
The adverb designate:
1640s, from Latin designatus, past participle of designare (see design
You can clearly see that the two words, design and designate came from the same Latin word designare that means to choose, designate or appoint.
The word design seems to mean "people who are chosen/appointed/designated", but it is not listed in the dictionary because its usage outside legal documents is rare. Personally, I found its usage only in legal documents, but I might be wrong.
As the noun assign was used to mean assginee, the word design should be interpreted in the same way even though we can't find the words in the dictionary.
As commented above, designs are broadly used in legal documents and it should be understood from its etymology as explained above.
[Oxford Online Dictionary, Online Etymology Dictionary]