No MX-5 had a factory fit alarm system until February 2001 production (NB F/L serial number 204926 onwards) from then on it became standard fit. cars before then may have dealer fit genuine accessory alarms fitted. These were Cobra or Finishline alarms.
The first “factory fit” immobilisers were the dallas touch key type & were introdued around 1994. These I believe were not truly factory fit but fitted at Autocheck (MCL’s workshops) on arrival in the UK.
Transponder type (chipped key) immobilisers became standard fit from September 1995 production, NA VIN serial number 300001 onwards. Again, accessory fit alarms offered customers an immobiliser option before the factory fit system was introduced.
Japanesse imports, to date, do not have factory fit immobilisers.