[rmlew]

Are you certain you are not thinking of the SMS Fürst Bismarck, a never completed Mackensen class battlecruiser?
According to Jane's Battleships of the 20th Century, the Mackensen class battlecruiser and the Bismarck class fast battleship were quite different. They carried 350mm (13.8 inch) cannons, not the 15 inch ones of the 1940 ship. Also they had a displacement of 30,000 ton full load instead of the 50,000 tons of the later Bismarck.
You might as well be calling BB-56, USS Washington, an update of BB-47, the unfinished Colorado Class battleship in so far as they carried the same name and both had 16 inch guns.

[Oztrich Boy]

Are you certain you are not thinking of the SMS Fürst Bismarck, a never completed Mackensen class battlecruiser?

Absolutely

Baden to Bismarck: A Story of Stagnation

While there is a superficial resemblance between Bayern and Bismarck, the real similarities are under the skin. Both ships had nearly identical three-shaft machinery layouts
Magazine layouts were also nearly identical between the two ships, another sign of heavy borrowing by Bismarck's designers. Where they differ is in their bridge arrangement, but here also Bismarck's designers drew on earlier work. Werner Fuchs, the Germany navy's head of construction, demanded that all heavy ships share a similar appearance, thinking this would confuse enemy spotters. Features included a heavy foremast for the fire control director and a single large funnel. Bismarck's upper works, like that of Scharnhorst and the Admiral Hipper class cruisers, was based not on Bayern but on L28
Bismarck and Bayern also shared strong internal protection, and were well-built ships (though British engineers had been rather scornful of the Bayern class in post-war testing). Their armor schemes were very similar, with Bismarck sporting thicker deck armor but a thinner armored belt than Bayern.

The 1917 L20/28 designs would have even been a closer match to the Bismark in appearance, size, speed.