As correctly stated, as per Section 1(3) of the Industrial Employment (Standing Orders) Act 1946, it applies to every industrial establishment with 100 or more workmen. But the proviso to it adds to it and extends it further as may be seen below:
(3) It applies to every industrial establishment wherein one hundred or more
workmen are employed, or were employed on any day of the preceding twelve months:
Provided that the appropriate Government may, after giving not less than two
months’ notice of its intention so to do, by notification in the Official Gazette, apply the
provisions of this Act to any industrial establishment employing such number of number
of persons less than one hundred as may be specified in the notification :
So you can approach the State Government for certification of your Standing Orders by relaxing the employee limit criteria.
Incidentally if the unit is in MP, then MP Industrial Employment (S.O.) Act 1961 applies and you are eligible to get it certified. The applicable provision reads as under
SECTION 02: APPLICATION OF THE ACT
(1) This Act shall apply to -
(a) every undertaking wherein the number of employees on any day during the twelve months preceding or on the day this Act comes into force or on any day thereafter was or is more than twenty;