Tri-Spark coils have extra windings using heavier wire and a high grade core for improved spark energy and duration with the benefit of being oil filled like the original coils. Oil filled coils have improved heat dissipation and work better than epoxy filled as epoxy is not a great conductor of heat. They are lighter and are not susceptible to internal arcing, which can destroy epoxy filled coils.
The appearance and dimensions are the same as the later Lucas type coil (17m12, 17m6) Diameter 40mm. They have 4.4 ohms primary resistance.