There isn't much available, but I did manage to glean the following:
In the beginning, we had26th Coast Artillery Battalion, a part of Harbor Defence.
As the war progrssed, there was less dependency on the Coast Artillery for shore defence, resulting in the reclassification of those men whose units were re-designated, either assigning them to field artllery units, or, as in your grandfather's case, creating a new unit.
The 26th CA BN was activated 28 Jan 42, at Fort McKinley,MN;
it departed Boston P/E, 21 Aug 42;
arrived Iceland, 31 Aug 42;
it was re-designated 749th AAA BN (Semi-mobile), 28 Oct 43;
as 749th AAA, arrived England, 1 July 44;
arrived France-ETO, 25 Aug 44.
Campaigns were as listed on his Separation Papers.
I can't ID the ships his unit traveled on in the GOOGLE entry .
My impression is that these lists have been partially upgraded since I last checked.
You may want to check back on this.
This was probably a Separate (unassigned) unit, assigned to various senior units in the various campaigns on a as-needed-when-needed basis.
You may go to GOOGLE, enter , and by bringing up each Division, then scanning down the data to ASSIGNMENTS, you may locate the 749th, in one or more of the divisions listed.
I would appreciate it if you would let me know how you make out. OK?