I spent around an hour looking into what online info there is on U-37, including around a half dozen photos, and her history. Are you aware of any of that? U-40 only made two war patrols and was sunk in October of 1939, so has almost no history.

Both uboats were of only eight total built pre-war IX design, later called IXA after newer variants appeared. Five of them were sunk in the first year of the war, and another in 1943. The remaining two were made training boats. In 1944, U-37 became an experimental boat, and was fitted late in the war with a prototype Type XXI conning tower (Turm.)

Per Wikipedia: "Between August 1939 and March 1941, U-37 conducted eleven combat patrols, sinking 53 merchant ships, for a total of 200,063 gross register tons (GRT); and two warships, the British Hastings-class sloop HMS Penzance, and the French submarine Sfax."

And: "On 1 May 1941 U-37 was reassigned to the 26th U-boat Flotilla, based at Pillau (now Baltiysk, Russia) as a training U-boat.[2][6] She was transferred to the 22nd U-boat Flotilla, based at Gotenhafen (now Gdynia, Poland) on 1 April 1942, remaining in the training role.[2][6] On 1 July 1944, U-37 was transferred to the 4th U-boat Flotilla, where she was used as an experimental boat until the end of the war.[2][6]"

Photos of U-37 available online dated 1940 show the net cutter and forward deck gun still mounted. Since she still has those in 1940, and U-40 was sunk much earlier (less than two months into the war) it would be safe to assume U-40 still had her net cutter and deck gun when sunk.

Since U-37's combat career was so brief, I would assume the kit is meant to portray her in that "heyday." Online drawings show her in a camouflage scheme (and U-40 in a different one) without specifying time period, but again, my assumption from knowing her history is that the camouflage would be primarily from her combat period. I did find mention that, as a training boat, she received a yellow stripe on her conning tower.

One photo shows U-37 in drydock in 1940, but I am not able to make out clearly either the gun or net cutter, so there is at least this yard period where it could have been removed while still a combat boat. I found two undated photos, one--like the her 1940 dated ones--showing the gun and net cutter, and one that looks like it might well be pre-war. Her number is prominently painted on her tower, there is no AA armament, her deck gun is present, but there is no net cutter. I speculate that perhaps, as a training boat, might have retained the deck gun simply as a training tool, if for nothing else.

This is the info easily available online, and what inferences can be taken from it. Unfortunately, nothing terribly concrete, but also not a complete blank.

FWIW, HTH some?