This is quite an interesting find. It's also interesting that you recall your Grandfather being active in exciting militant republicanism once more. The 1930s was a defining decade for physical force republicanism. The IRA survived proscription by erstwhile comrades, Dev et all; a significant effort from within the movement to shift to the left, and embrace some form of social politics. Despite all this, the physical force strand of the movement succeeded, under the command of Sean Russell, and ultimately led to the S-Plan bombing campaign. 1939-40.
Moss Twomey was COS '26-'36. His papers are deposited in the UCD Archives, and contain a wealth of coded IRA correspondences. These were decoded and published by Thomas Mahon and James Gillogly in a book called 'Decoding the IRA'. There may be some correlation between this work and your Grandfather's copybook. On the other hand, there might not be, but it could be a sound starting point for you.
Good luck with your investigation.