A brief internet search showed that Silvercraft Limited of Bray, Ireland was in business from 1963-1998. Don't know what kind of unsolicited goods they were shipping to the USA, but it must have had some weight to be uprated in the registered postal stationery envelope.
If this were mine, I would remove the adhesive tape.
It appears to be an ordinary Irish postal stationery registered mail envelope with extra postage to cover the cost of it containing something heavy and that was sent to the US by regular airmail in 1969. It went through customs as did most every piece of mail that likely contained something. Most all of the cancels and markings are very poorly inked. This is the kind of thing I would expect to find in a dealer's dollar box.