[Python] Finding all valid eight letter words by adding one extra letter from seven letter...

I have several long list consisting of thousand of words sorted by length, one list for every word length. For every word of length n in a list I would like to generate all valid words from the list with words of length n+1, that is just by adding one extra letter.

By trial and error I have found something that does the job but it is messy and runs for ages as I am very new to Python. I struggled a lot with all the variations with duplicate letters... What is a better and more efficient way to get the the job done?

SevenPlusOneList = []
for word7 in ListOfSevens:
SevenPlusOne = []
SevenCount = collections.Counter(word7)
for word8 in ListOfEights:
EightCount = collections.Counter(word8)
NotSharedLetters = {k: SevenCount[k] for k in EightCount if
k in EightCount and SevenCount[k] != EightCount[k]}
if len(NotSharedLetters) == 1 and len(EightCount) >=
len(SevenCount) and (len(EightCount)-len(SevenCount)==1
or list(NotSharedLetters)[0] in SevenCount):
SevenPlusOne.append(word8)
if len(SevenPlusOne) > 0:
SevenPlusOneList.append(SevenPlusOne)
else:
SevenPlusOneList.append('-')

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