Given how early in the war she was lost, the U-501 should have been built with a single 3.7cm Flak SK C/30 semi-automatic on the lower deck behind the bridge and a single 2cm flak 30 on the bridge itself. Sometimes however, the single 2cm was deleted on the bridge, but given her build date it would be safe to say this would be her armament. She would have had the Turm 0 tower, so the turm 4 in the Revell kit will not work if you are looking to build her as she was sunk. The turm II did not come about until December of 1942, and the Turm IV around March of 1943. The Turm I (U-193 and U-553 are the only two so far known to have this tower) and Turm III was for the VIID mine layers.

That is my best geuss based on the information I have in front of me. A picture of her leaving before the last patrol would help even more. However, as I have discovered, many websites miss-label U-boat pictures. I have seen the U-550 sinking pictures labled as U-1230 and so on. Hope it helps and that someone knows for certain one way or the other.