I've been doing this for a long, long time, and that's actually quite untrue. The difference was, back then, that appraisals were not regulated, so people would just slap a number on them.
But look at it this way--in 1970, or 2015, you walk into a gallery on Park Avenue and buy a painting, or you walk into an antique store in Austin TX and buy a Chippendale side table. Whichever, you pay $20,000 for it.
Six months later, you need some cash, so you want to sell it. You're not a dealer with a store or an established clientele. You have three choices: you place a classified ad, you put it in an auction, or you give it back to the dealer on consignment. Either way, when that sale is done, you will be receiving somewhere between 30% and 60% of what the original dealer would get, or what that same dealer would get that same day. It's not at all feasible for an individual to get anywhere near what a dealer would get, on the same day, for the same item. That's why nowadays appraisers are required to be very careful to know and state the reason for the appraisal, so it can be appraised in the correct market.
Often, as an example, I will show people the auction record for a painting, then the dealer (who bought it at auction) listing the exact same painting on their website for 2 or 3 times the price. It's the dealer's store, clientele, reputation, and established expertise that allows for that.
That said, I'm not going to be able to get you the information you're looking for based on that serial number, so I'll opt out.