U-441 / U-Flak 1 „Flakfalle“

I personally would second the 'swtich' the number to a 6 being a destroyed aircraft, and a 1/2 being it continues the attack to keep with the pattern of the high rolls being the desired outcome for the U-Flak's attack. Though again that may just be personal preference.

13 points also seems rather high for a 1 hull sub. Comparing it to U-510 for example: U-510 has better torps, same SAs (except no AA ability), and 2 hull points for 2 points less than this unit.
Personally I would see this as somewhere around an 8/9, maybe 10 point unit at most. You're essentially giving a 2/3 chance at an aircraft abort/destruction for the trade off of less torps and less hull. With the unit being less of a threat to enemy ships and more of a threat to enemy air, your opponent is likely to prioritize going after your other more punchy/less defended subs first.

IMO I would make some adjustments to the U-Flak SA, and again these are just personal musings on the fly.
I would have something similar to Surcouf's SA, because the sub is for the purposes of it's SA, on the surface conducting an anti-air attack. Thereby it would be vulnerable to surface unit fire. Or more vulnerable to air attack. So a modifier similar to Submarine Corsair or a penalty to armor for a failed attack (a roll where the enemy aircraft continues their attack unaborted/destroyed) would be understandable.

I realize that makes it a pretty complicated SA overall, and IMO the trade-off for the AA attack doesn't warrant such a large point investment. I really enjoy the unique-ness of the U-Flak idea, both in-game and in practice, and do see it's benefit. Especially if you involve any stacking limit houserules where this could potentially provide air cover for multiple units. Also by having this sub be less of a target, you're pretty much guaranteeing that you'll have a Wolfpack SA around for a while for any other subs that do have the same SA to benefit from this unit.

An SA to increase sub limit +1 would be very useful and powerful imo, because a 2/3 chance at an abort/destruction is pretty good, and made better by actively being able to screen another submarine that may be able to hit harder. Downside is potential ASW suppression is twice as effective, but it would add a lot of punch to this unit.