Generally the difference is the turns ratio and/or the
impedance that the coil is designed to work into. The
last IF coil is usually intended to drive a diode detector
and has a lower output impedance than the others.
If you are building your own IF strip you can design it
to match the impedances of the transformers you have. I'd
start by driving the low impedance side with a known
signal level and measuring the output side with a scope
to get an estimate of the turns/impedance ratio. Typically
these are designed for bipolar transistor circuits, with
impedances suitable for collector and base circuits.
For example, if you go to this page from the Mouser catalog:http://www.mouser.com/catalog/635/982.pdf
it shows many of the transformers available from Xicon.
Most have primary impedance in the tens of thousands
of ohms and secondaries in the 500 to 5K ohm range.
One approach you might try is to use the transformers
to step up the voltage and FET stages as impedance
transformers between them: apply the signal to the
secondary and feed the primary to the gate of a FET.
The secondary of the next transformer goes in the drain
lead of the FET. Because of the low drain load the FET
itself doesn't provide much voltage gain and the circuit
can be quite stable.