thanks for the pix and agree that apart from the small amount of cutting, this is a moulded piece. Mould produced bottles do in fact go way back into the C18, many of them often copying the decoration seen on cut decanters. The body shape on this one is unusual - something of a cross between a ships decanter, a mel (Scottish probably) and a variant on the shaft and globe - it's certainly not a common shape, but perhaps easy to manufacture. The long shoulder slope from the neck to the wide waist is distinctive, though this piece is definitely modern - which means almost certainly you will have the stopper that it left the factory with. Non-original stoppers are a pain, especially if like some of us you have a habit of buying in charity shops and bric-a-brac outlets - they're easy to spot due to the fact that they wobble in the neck of the body - you might try to re-negotiate the price if you sound knowledgeable and point out the bottle and stopper are 'a marriage'.
The stopper here is probably some minor variation on the mushroom shape. Try to avoid bottles that are cloudy on the inside - this can sometimes be removed with the use of Magic Balls - copper coated steel balls that are placed into the decanter, together with a little water and perhaps some cerium or aluminium oxide, but results are unpredictable. Stand and shake for a while, then removing all the moisture from some decanters can be long winded job. You might now want to look out for a stopper and bottle both with the same matching No. - there are a lot of pre-owned decanters out there, too many with the wrong stopper probably, but a massive choice. Look at colour of the glass - too white and it will almost certainly be modern - a dull leaden colour may well indicate some decent age. and most importantly - buy what you like, always the best policy. best of luck.