Given the urgency to get the new elektroboats into service with the war winding down, the Germans came up with a different method of producing submarines in order to make better use of their shipyard resources.
Typical U-Boats would be built from the start occupying a single slip which could take up to 18 months for the larger ones. Since this timeline wasn't acceptable for the new U-Boats, Speer used Otto Merker's experience in mass production to create an alternative possibility.
The U-Boat would be built in pre-fabricated sections in parallel, distributed among land based factories around the country. These prefab sections would test fit a "master" copy of the next section and then would be transported via rail or water to the shipyard where it will occupy the slip only long enough for final assembly. Instead of occupying a slip for 12+ months, the submarine would only occupy the slip for around 2.5 months while being assembled, with a total production time per vessel of around 6 months.
In this way the bottleneck caused by shipyard resources (limited number of slips) is greatly reduced, and (in theory) you could build 5-7 times as many submarines in the same timespan after production capability was fully ramped up and organized from factory all the way down to shipyard.
Historically this was not without it's own issues. Production schedules and deadlines combined with the strain on German industry under heavy attack late war meant that the quality of construction could be subpar with the modules sometimes not meeting the fine tolerances required to fit the next one, causing delays.
However, the important part here is that although historically the reason this production method came about was due to the urgent need to expedite the development of this particular submarine, there's no technological reason they could not have applied these methods earlier -- the methods used are just as valid in 1939 as they were in 1944... it's just nobody thought of it. There wasn't a need and necessity was the mother of innovation. What if they had?
Here's where the timeline differs -- say Germany, in an effort to secretly ramp up militarily, began developing this method of production in the mid 30's, building prefabricated submarine sections in factories by the late 30's and keeping them concealed and ready (whilst increasing training for submarine crews). When it became clear war was looming, everything was ready except the final assembly in the slip and outfitting.
Instead of entering the war with around 60 submarines as historical, what if they entered the war with triple this number? Further, entering the war with the infrastructure already in place so that production continued to accelerate?
Would these additional submarines have been influential this early in the war during Happy Time? How would the allies have responded regarding ASW?