Sounds like an update/backdate rule SCCA uses for SOLO II. if he could lost the exact wording/rule we could help him find the competitive advantage.
Per the: Regulations Of Competence For The Categories Of Cars Of 1.6 Liters With 16 Valves - 2022 STT (SUPERTURISMO TAPITIOS)...
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21 TECHNICAL DEFINITIONS
21.1 Original. Where the term "Original" is mentioned in the Technical Regulations of each category, refers to manufacturer's parts.
(skipping ahead...)
22.10 GEARBOX
22.10.1 The housing must be original. Updating is allowed. (NOTE: THIS ALONE *SHOULD* ALLOW ME TO USE AN ST200 IB6 W/LSD BUT I WOULD HAVE TO "LOCK-OUT" 6TH GEAR-BY RULE)
22.10.2 5 Speeds and reverse. If it has sixth gear it must be locked out.
22.10.3 Original manufacturer gears are allowed without modification. No welding is allowed.
22.10.4 Spur(straight cut) gears are prohibited.
(skipping ahead...)
22.12 DIFFERENTIAL
22.12.1 Original must be used.
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As you can see, there is some "original" room to play with IF an OEM LSD was/is offered in a Fiesta gearbox..."original" meaning anything the manufacturer offered.
Thanks guys for your help on this. Later on, there may be other issues where I need some help in finding the right wiggle room.
DJ