With the T3, this is an impedance matching transformer and from the info given it is about a 5000 ohms plate to plate impedance to give an 8 ohm output impedance. The tapping for 4 ohm would be at about 55 turns at a guess. The turns ratio suggests an impedance transformation ratio of about 450 or so.
The construction of this transformer is probably using a thinner core lamination material compared to the power transformer.
The design is obviously very old because of the electromagnet field winding. This stuff was obsolete by about the 1950's or earlier when ALNICO permanent magnet material was developed.
The project seems to be hung up on whether the transformers are faulty or not. When you ask about 'power ratings', I sense you are talking to a 'new transformer supplier' and if this is the case you will likely get nowhere fast. For a restoration, and IF the transformer is faulty, a rewind is suggested.
I note you are in OZ as I am, and I can tell you that what is available in Australia today is nothing like what it used to be. Getting the kind of components you want, used to be dead easy, but now all you get is frustration. Drongos in supply houses work out of catalogs and its like talking to a brick wall. If you need another audio transformer, get the original rewound. When you find someone to do this job, you will find he is an old guy about 70, who can do stuff.
I restore and collect pianolas, and I am aware of the Seeburg company because they made orchestrions in the 1920's. They've been around a while. Much of this old stuff is getting hard to renovate and guys have to be inventive. In the US, there is a huge industry of backyarders who can do anything.
hope this helps.