For the application of the API monogram, it is dependent on the date of manufacture. Date of manufacture is defined in API 6A (section 3.1.27 of 21st edition) as "Date of manufacturer's final acceptance of finished equipment", which implies that it must be once the equipment has been fully manufactured and passed all inspection and testing. For the application of the API monogram, the shipping date and contract date are both irrelevant.
If the date of manufacture (using the above definition) is after November 1st, the equipment must meet API 6A 21st edition in order to apply the API monogram. If the date of manufacture is October 31st or earlier, the equipment may be monogrammed if it meets either 20th edition or 21st edition.
Whether the customer will accept 20th edition after November 1st is a contractual issue that you should clarify with your customer. You can still manufacture to 20th edition after November 1st, as long as you don't apply the API monogram and as long as your customers accept it.