First of all, I am not a lawyer and am in no way trying to provide a legal perspective here! This is purely a personal reflection which I hope may be useful.
If the addendum explicitly contains language that says it replaces a previous contract, then for most practical operational purposes may not need to refer back to the original contract.
If the addendum doesn't contain such language, whether or not you need the original contract will depend on what you're trying to achieve. If you want to understand the complete terms of the contract, the addendum will be unlikely to tell you all that you may want to know. If you just want to prove to a vendor that you have executed a renewal, then the addendum may be enough (assuming that the vendor doesn't question the validity of the addendum or whether an original contract in fact exists).
In terms of how you record this with contract records in FNMS, a suggestion would be:
1. Create a contract record for the addendum.
2. Create a 2nd contract record representing the master/original contract which you believe exists but have not been able to find. Add some comments or an attachment on that record noting that the location and details of the original contract documentation are unknown.
3. Set the "Master contract" field on the addendum contract record to this 2nd contract record.
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