The U1104..
has a different armature to the 1346/A/1354/A. The 1334/A or U1012 armature is not a straight swap because the shaft is longer and designed for a machine with a round belt pulley, as opposed to the flat belt pulleys of the U1104 or U1036. Also, the U1104 armature is different to the U1036 one; the U1036 is bigger in circumference across the laminations..and the commutator is also bigger. I have swapped the U1036 type with U1104 and vice versa. It's not so bad going from the large to the small type. The other way about is no so advisable, since the brushholders are pretty close to the commutator.
The fans on the motors for the flat belt machines were also different. The centre of the fan boss didn't protrude as much. Early U1036s, with metal fans, had removable plastic inserts, unlike the later plastic ones. 1334s had metal ones originally with non-removable plastic inserts; then there were the replacement plastic and metal fans.
I've rebuilt loads of Junior motors but have never thought it feasible/physically possible to swap armatures between flat & round belt machines.
Stephen