Ooohhh. That is indeed an oldie. The style is absolutely from the 60s, IMO. From the looks of it, it may have indeed been a custom job and hence the SN stamped into it.
Very nice find!
If it were me, and I could get 1200 for it, with a 500.00 investment - it'd be gone yesterday.
(WARNING AND FULL DISCLOSURE - THIS IS HOW FLIPPING TACK AND SADDLES CAN BECOME AN ADDICTION ON PAR WITH GAMBLING. Ahem)
Locally, those go for about 200.00 - so don't tell your buyer! (That said, it's a buyer's market where I live - throw a rock and you trip over a nice western saddle for sale on the cheap).
I do hope the offer is local - be careful selling online, don't ship without money in hand from Paypal first, and do follow all the seller protection rules so you don't loose the saddle, the money, and your shirt if they try to defraud you. IF you're shipping it, absolutely have the buyer pay for it and purchase insurance so they can't dun you for money if the shipper destroys it en route or conveniently 'loses' it.
Now - it being the age it is... and I have some that old that I use as autopsy and display saddles... it may well fit your horse, given his size. I think the 14 hh range sized QH's are more in line with the 'old' build. I have one that's a Doc Bar grandson, and a saddle from the 60s fits him like it's made for him... we don't ride it because it's so dad gum uncomfortable in terms of seat positioning. Very... weird... compared to modern roping and barrel saddles for us. LOL
TL;dr: SELL THAT THING!
"We are here to laugh at the odds and live our lives so well that death will tremble to take us."