The following account has been drawn entirely from memory since all related references are now long gone.
Output transformer interleaving comes in three basic structures.
First the hybrid type: You use three bobbins. You split the primary in two equal parts & connect it in series. You insert a third bobbin carrying the secondary between the two & put the whole assembly in the central limb of the 'E' core section - this is the classic 'balanced sandwich'. Using a 'C-T' core is practical only at power line frequencies since the magnetic path between windings becomes too long causing an unacceptable degree of hysteresis.
In the second type, you are winding the primary linearly in layers. You insert insulating paper between each layer. As you approach half the quantum of turns, you start winding the secondary in alternate layers between the primary; you then wind the balance of the primary over the entire assembly. This is the 'Williamson' type of output transformer. If you want to be finicky, then use a slightly heavier gauge of wire for the final section of the primary; if you prefer to cheat, then insert a small ribbon resistor in series with the shorter 'half'.
The third type is interleaving the core. This minimizes the gap, so increasing the permeability & consequently reducing the lower cut-off frequency of the device. The downside of this is that it cannot accept a DC component in the signal; thus it is only capable of handling balanced outputs, such as push-pull. Tapping the primary to drive the screen grids of the output tubes tends to smooth out non-linearity at the transition points; this variant transformer type is referred to as ULTRA-LINEAR. A five section interleave will provide good results with this type of construction. The five sections should be proportioned as follows:
1/4 Pri., 1/2 Sec., 1/2 Pri., 1/2 Sec., 1/4 Pri.,
where the combined mass of copper occupies a minimum 4/5 of the entire core window space leaving the remainder for insulating material; moreover 5/8 to 2/3 of the copper mass should be primary winding. As regards the first two aforementioned structures, 1/2 to 3/5 of the winding aggregate should be primary.