I am in agreement with @user3697176's answer, however would make two changes:
Oct 21 spr of Brook St. Chester
Cert R. G. Chester Sept 1903 - 8a 843 5169
- spr = spinster
- Cert = Certificate
- R.G. = Registrar General.
The last line is obviously the marriage reference in the GRO index (as per FreeBMD):
I also agree that the witness's name is clearly written Ewin, but that is not to say it might not have a been originally Cowin. Errors like this may have crept in at any point between the original marriage register being signed, a copy being made to send to General Register Office, or the transcription being made for this index card.
Lastly, there should be another index card under Cowin's name. I couldn't find it on a quick search, so it may have been lost. But there was definitely one made. It would be useful to compare what is written on Cowin's card.