Attached some pages from 'Les Grenades a main, cours munitions' de l'armée française
the GR MA FUM PH OF 59 did not use the 2,5s fuze according the tech. docs.
Also, no vent hole was present in the base.
I slightly misread the question (tought it was about the OF version), so i also added the pages of the GR MA LAC 59
The GR MA LAC 59 did use the 2 or 2,5s fuze.
In the first picture of the manual, there seems to be an option to connect multiple grenades, implying that the bottom does have a venthole/screwhole, but in the separate drawing, it is not mentionned....
Could be a change from earlier grenades (the document is from 1980, so alterations to the GR MA LAC 59 are possible)