U780 to U791 were planned but never built:

U-Boat Emblems of World War II 1939-1945 Georg Hogel page 192.

unknown subs
 

According to Conway,s all the worlds ships 1922-1946 printed in 1980. There is no sub type U1200. U1200 Was a type VIIc/41 boat. launched 1943-1944. O which 70 units were completed. More than half the completed boats survived the war.
It is believed that U1200 did not survive the war.

U791
 

U791 was launched in 1942. listed as ex-V300. Conways all the worlds ships 1922-1946. Printed in 1980.

U791
 

Type VIIc/42 boats 176 unites were ordered but never commissioned. Correct is U783-790.