Hello Falo,
Well spotted on the lack of containers. According to page 10 of my article "List Of U-Boat Modifications & Identification Features" the four containers (sometimes fewer) were a feature that were added in late 1944. I have been unable to provide a more precise time date than late 44. I reckon the UiF photos of U 481 in the summer and autumn of 1944 show the boat before any life containers were added to the foredeck.
One photo (probably from 1945) shows U 481 with one singular container on the foredeck. I presume that was added sometime in late 1944 or so.
Cheers,
Dougie