API 6A and 16A Monogram stamp 1

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.

Thank you jmec87, that was my understanding as well, just wanted confirmation I understood it correctly.

Here's the question, what if you already have equipment that has been manufactured and test that you have already stamped with your 20th edition stand, can you sell it after Nov 1st with the stamp on it since it was manufacturer and tested prior to of do you have to remove the stamp?

You do not have to remove the monogram if the manufacturer's final acceptance of finished equipment (i.e. completion of all required manufacturing, inspection, testing, marking, and whatever you define as your final acceptance step) is completed before Nov 1st. Note that the manufacturing date (month and year) has to be marked on the equipment along with the monogram.

There is no limit from API for how long after Nov 1st you can sell equipment monogrammed to 20th edition, provided that the date of manufacture of that equipment is before Nov 1st.