From a BR Training Booklet:

AC203
This machine is similar in construction to the earlier AC8 and consists basically of a field coil assembly (rotor) which rotates inside a stationary set of generator windings (restator) but with the major difference that rectification of the generated alternating current is carried out by a built-in rectifier bank of 6 diodes (see fig. 1).

While again the field coil is fed via slip rings no seperate provision is required for initial excitation as the d.c. supply for the field is supplied direct from the alternator and put via a seperate built-in auxiliary rectifier bridge of three diodes sharing the same heat sink as the six main diodes (see also fig.1)