Photos above represent the original style spark plug connector for MGA. Photo below is an original New Old Stock Lucas part with original packaging. The nose end of the connector has a split brass sleeve to snap onto the spark plug top end. The tail end of the connector has a socket to accept the HT wire. Inside the socket is a small coarse thread pointed screw, like a sheet metal screw. To install the connector on the wire, wet the wire jacket to make it a little slippery, then screw the terminal onto the wire. As the screw penetrates the core of the wire it will expand the insulation for a tight fit in the socket, making it water tight and very secure. The outer housing was originally molded phenolic with "LUCAS" embossed on one side and the manufacturer's part number embossed on the other side. Printed logo is not as original.

Hidden in the middle of the connector there is a resistor to make the connector into a suppressor (as in reduction of electronic interference for a radio). The Service Parts List makes a point of calling the part "Suppressor - ignition cables", as was used on all MGA and early MGB. Any resistor may degrade over time, so it bears testing occasionally if you suspect a faulty spark. Expect resistance to be in the range of 3000 to 5000 ohms.

Part numbers:
3H1422 - Suppressor - ignition cables (original BMC number for MGA and early MGB)
78106A - Lucas number
60460666 - TRW LIMITED T/A TRW AUTOMOTIVE AFTERMARKET OPERATION
ASB601 - TRW LIMITED T/A TRW AUTOMOTIVE AFTERMARKET OPERATION

Embossed parts available from Brown & Gammons, ukmgparts.com - (* see below)
Printed parts available from AH Spares, Amazon, os eBay SC Classic (southernspares).
Unknown quality parts available from auto-part.com, EnglishParts.com, MGOC Spares, Moss Europe, Moss USA (171-620), somerfordmini.co.uk

Some original applications for part number 78106A:
MG, MGA 1955-62
MG, MGB 1962-1967 - G, GA, and GB engines
Morris Commercial, LD1 + LD2 1954-60, LDO 1956, LDO1 + LDO2 1957-60, LCFO 1958-59, 3KNC 1956-59
Nash Metropolitan, 1954-1960 (When Fitted)
RELIANT, 6 cwt. Van and Truck 1954-55, 10 cwt. 3-wheeler Van 1951-56
RILEY 'One-Paint-Five' 1958-60 (W/F), 'Two-Point-Six' 1958-59 (W/F), 'Six Ninety' 1955-59 (W/F)
WOLSELEY 'Six Ninety' 1955-59 (W/F), 'Fifteen Fifty' 1957-59 (W/F), 'Fifteen Sixty' 1959-60 (W/F)

In case you might be wondering what is inside the suppressor type spark plug connector, the cutaway at right is a (non-standard) connector with molded plastic shell and shielded nose. The little coil spring goes between the screw and the resistor to make the electrical connections tight and (hopefully) reliable.

These terminals are intended for use with solid core High Tension wire, as you do not need resistor terminals and resistor wire at the same time. The resistor in the HT circuit is commonly used for RF (Radio Frequency) noise suppression to reduce static on a car radio (being picked up by the antenna). Modern electronic radios should have static suppressing circuits built in so the HT ignition resistor is usually not needed for that purpose. Resistor caps or resistor wires are likely to reduce long term reliability of these ignition parts. If you don't need them for RF suppression I would recommend not using them. For newer cars, especially those built after 1980, the HT resistor parts may suppress RF interference for some other on-board electronics, like the engine control computer.

* - On 29 August 2016, Gary Starr in Illinois, USA, wrote:
"Brown and Gammons Lucas plug connectors .... Since the last time I drove the car I have had 3 of them go bad. ....
https://www.ukmgparts.com/product/twin-midcat-1-twin14-ignition/plug-cap-3h1422