[1880nonsports]

or auction house that handled the original book and likely any pages would have known whether the cards would soak - either having tried it or knowing second hand. PERIOD. A consignor who is likely unfamiliar with such ephemera is another thing.
As to when someone might have glued such cards onto the pages is anyone's guess - a leap of faith given the limited information. I would suggest it was likely NOT close to the time they were issued but more likely a second or third hand accumulation of cards. Doubtful it was done recently in an attempt to create a deception. Of course the devil must be given his due - I've seen much worse.
As for the OP taking his shot - most here that have done any volume of buying/selling have taken shots only to be disappointed at times. Count me among them on one or two similar transactions. I have also come out on the positive side of a few including a Goodwin Champions album. I empathize (not that it does much for the OP) - just want to suggest he's not alone. The more EDUCATED shots a knowledgeable person takes - the greater the likelihood of success - at least that's the proverbial carrot.
The "cards" still represent a nice selection depending on one's level of expectation. Life usually gives you the test and then the lesson. As long as we can learn from what we've done - that's the rub...............