The plan is one from a set of dockyard / line drawings - to date I have seen few of these and consider this one to be quite unique quality of the drawing speaks for itself.
The type XXIII was a small Coastal boat desiogned for specifically with submerged running in mind , she had a diving deopth od 100meters, although had been test dived to 150 and a destruction dive depth was found to be 250 meters. ( Carried out in Norway).
The boat had only two torpedoes which were loaded from the exterior directly into the tubes .
Boats 2321 to 2371 , 3001 to 3035 , 3037 to 3041 , 3044 and 3501 to 3530 were commissioned ( from Jak Showell).
Boats 2321 , 2322, 2324, 2326, 2329 and 2336 made operational patrols , a further 12 boats were in Norwegian waters at the end of the war.
Operational successes
U2321 sank SS Gasray (1406 grt) off St. Abb's Head.
U2322 sank SS Egholm (1317 grt) of Berwick.
U2336 sank SS. Sneland (1791 grt) off Dunbar and SS Avondale Park
(2878 grt) - the last recorded sinkings by U-boats in WW2.
U-2326 was used by the Royal Navy as N-35 before being given to the French navy in 1946 - she sank off Toulon on 06/12/46 with the loss of 17 lives - she exceeded her safe dive depth.
Of 378 boats ordered 62 were completed , they displaced 234 tons surfaced and 258 submerged.
Above info from.
"German U-Boat losses in WW2" ( Niestle).
"German U-Boat Operational Careers" Vol.2. (Wynn).
"The U-Boat- The engineering and technical development of German Submarines" ( Rossler).
Currnet issue of "The Armourer" a photo of U-2336 surrendering at Dundee.