The letter and number after the part number (A5, A8) are manufacture identifiers (who made the part), and can also be the difference between Mercury and Quick silver. So don't give it much thought
This could be some very bad advice!
For this engine 0D808410
18643A7 is the coupler you should use, this p/n was 18643A2 and then 18643A4 before it's latest number.
If your serial number was above 0F000001 you would use 18643A5
If you had a Bravo the coupler number is 18311A17
18643A8 was used on jack shaft engines with a one piece rear main seal and is NLA
I'm pretty sure these are all different parts, I don't think MerCruiser would stock the same part under multiple numbers.
The difference between Quicksilver and Mercury Precision Parts is Q is used for Quicksilver instead of A