If the bonds are in a safe deposit box which is not rented to you and to which you do not possess the key anyway, and you do not have a letter or other document
from your grandmother in your possession saying that she has given you the bonds
(which you can collect from a safe deposit box in Bank X in City Y in State Z),
you are SOL. The bonds are part of her estate and do not belong to you.
that any bank or even the Treasury will cash bonds registered in your
grandmother's name knowing only the numbers, or even the actual bonds
themselves, unless you have a letter in your possession from your grandmother
saying that she has gifted the bonds to you. You might be
able to follow the procedure for replacing lost US Savings Bonds if you
have a letter and the bond numbers, but now we
are getting into legal details that are inappropriate for this forum.
I don't like to say this, but your story does not pass the smell test.
In view of the revised question, I would say that @DJClayworth's comment
on the main question is probably the best course to pursue. You could
try looking at Form PD F 1048 (available through the TreasuryDirect web site) which
allows you to ask the Bureau of Public Debt to re-issue lost or destroyed
bonds. But the Form may ask you to affirm under penalty of perjury that all
the statements you make on the form are true, and you have stated that the
bonds are neither lost nor destroyed, just inconvenient to retrieve, and
you don't really need the bonds to be replaced for the purpose of holding
them as investments, possibly in your personal safe deposit box, but to
cash them in.