Looks to me like a genuine forgery its been varnished over to give the impreshion its old but the cracks are to consistent.
Might be a goose chase but: https://www.ancestry.com/search/categories/bmd_marriage/?name=William_Rashbrook&gender=m
Unless you have one of his from when he was reeeely decrepit and signing in capitals .... he seems to write his name in full and with both upper and lower case
I can't resist delving ... Has been a useful trait for work How about initials and then where it is, as in, Ashbrook?