Late post, but, my block in my SD4 car is cast with 2.5 SD on it as well, it is an early block from early 1985:
From what I can see based of others photos, and the SD4 build guide. The blocks originally came machined off of the factory Iron Duke line. they didn't ship come with 1/2 head and main bolts. The cam tunnel only had 3 bearings like factory, and they were journal bearings, Those blocks are marked 2.5 SD below the distributer. Later blocks (I cant find the date change) came from GM with 1/2" main and head bolts, and had 5 journal cam tunnels with roller bearings, those blocks are marked Super Duty under the distributer.
EDIT: explained markings better.
[This message has been edited by FieroWannaBe (edited 08-17-2020).]