Early internally regulated unit, showing the separate plugs for the + and -, and the IND and B+ terminals. There are two IND spades, although one (nearest the designation) is not as clearly visible as the others. The brown/yellow comes in to one of these from the ignition warning light, then loops back into the harness and out again to the other, possibly to protect the alternator if that plug should become detached from the alternator. There may be a black earth wire on the '-' terminal. The embossing on the rear cover implies it is a common item for 15, 16 and 17ACR units, but 17ACRs on the MGB didn't have this two-plug arrangement. Photo courtesy of Craig Locke.
Plugs fitted, there is an 'interlock' between the two so that the ancillary connector cannot be removed or fall out unless the output connector has already been removed, probably to protect the alternator from excess voltage/current. I doubt it prevents the plugs being put back in the wrong order though, and the engine run with only one in: (Martin Ward)
The various connection arrangements used over the years:
This from Brown and Gammons subdividing the 'European' termination into Type A and Type B:
1972 alt plug in the Euro style, showing a single large brown and the smaller brown/yellow (photo David Bolton)
15/16/17ACR machine-sensing diode pack for this style of plug, with two large (9.5mm) output spades connected together:
The Bee's (1973) harness plug, showing the large output wire in the middle, and a smaller gauge brown wire on the other large spade, plus the brown/yellow. In theory this should be battery-sensing wiring needing a different diode pack - still two large spades but one of them for the battery sensing wire.
This is a battery-sensing diode pack, but to the later design with two standard-sized spades, the lower one with chamfered corners. The BSH connector thin brown connects to this spade (B+), the brown/yellow to the unchamfered standard spade. Note that the central large spade is about double the width of the other spades, which makes it 12.5mm, despite the description stating it is 9.5mm, which is the size of the large spades in the Euro connector. Image from JCR Supplies as being for an 18ACR alternator:
Bee's alternator showing the two large and one small spade i.e. 'European' of either type A or B. Definitely a replacement unit as the voltage regulator is a 21TR with a 1987 date code. This appears to be an A127 with metal back plate, probably 45 amps. Also available with spade terminals at 55 and 65 amps, and 70 amps with stud output terminal.
Looking straight into the connector, the two large spades do seem to be connected together i.e. a type B
An A115-45 alternator from a Metro, the two large spades are definitely connected together here.
Image from Moss USA showing a 'European' or type B three-pin connector with two large output spades and the smaller Indicator spade, but also two additional spades which aren't used.
Peter Mitchell's 78 plug also showing one large and two standard spades as in the 'BSH' connection above.
October 2014: Converting between the various connection arrangements. As shown above 4-wire alternators have two large spades in one plug, the '+' terminal is the output, the '-' terminal is an earth terminal (unused). The other plug has three terminals - two IND terminals side by side and the B+ or battery sensing terminal below them. 2-wire alternators have two large spades and one small, both the large spades are connected together and are outputs, the smaller spade is the IND terminal. 3-wire alternators have one large spade and two small, with one small spade having chamfered corners. The large central spade is the output, the plain small spade is the IND terminal, and the chamfered small terminal is the B+ or battery sensing terminal. When converting from the 4-wire discard the loop of brown/yellow that goes between the two IND terminals. When converting to the 2-wire optionally connect the thinner brown to the second output terminal to reduce volt-drop and improve system voltage. When converting to the 3-wire all three wires must be connected as shown.
See also this PDF from Moss Motors on how to convert from the 5-wire 2-plug system to the later 1-plug system.
V8 (1975) AC-Delco alternator, same 3-pin plug, but this time only two wires (normally - the second output wire is my modification giving a boosted voltage supply to the cooling fan relay for improved performance). Two large spades in the alternator both joined together i.e. both output spades plus the smaller Indicator spade.