AFAIK, nearly all TR4A were negative ground from the factory, and again nearly all equipped with a generator. I just checked Bill Piggott's latest book, and he seems to agree. Apparently the Lucas 11AC alternator was part of a "police package" (!) that was even available on the TR4 (and would therefore be positive ground). Perhaps this is the reference to a positive ground alternator you recall, Andy.
The 11AC alternator, unlike most (later) alternators, is relatively easy to configure for positive ground operation. It is externally regulated, so the only internal polarity-sensitive components are the diodes. The diodes are pressed into heat sinks, so all it takes to configure for positive ground operation is to install the "positive" diodes into the ground heat sink and vice versa.
My early Stags have 11AC alternators, and the external control circuit is oddly complicated with a relay cutout mechanism in one box and the actual voltage regulator in another box. Apparently, Lucas felt the rotor might overheat with full field current and the engine stopped, so the cutout box contains a resistor that supplies a reduced field current when the ignition is on; plus a relay that operates on a low voltage AC signal brought out directly from the windings, to only apply full field current when the engine is running.
Altogether now ... Emm Eye See; Kay Ee Wye; Emm Oh You Ess Ee !