That schematic is the same one shown on Fig.4, page 171 of the 1961 Mullard manual.
The transformers are identified in the text as Colne 06005 (for T1) and Colne 06006 for T2.
The equivalent Hammond transformers are listed in the back of the manual as 57318 and 57319.
Maybe someone has an old Hammond catalog that lists the specs?
Text also states that this amp is the same as the one shown in Fig.13, page 160, (but with the preamp added).
Fig.13 has some further transformer info:
T1 is 3.5:1+1, Rp < 130 ohms, each half of the tapped secondary has Rs < 40 ohms.
T2 is 3.1+3.1:1, each half of the split primary has Rp < 1 ohm, Rs < 0.2 ohm.
Speaker is 3 ohms.
The OC81D driver and OC81 matched pair were supplied as a set by Mullard.