Oldest beers with unchanged recipes...

I'm sipping a Westmalle Dubbel (currently 3 for £6.00 in M&S) a beer that has been around for a long time and, I think, unchanged since 1956 (was the original brewed from 1856?). Definitely an older beer (and I've never tasted a newer dubbel like it). I've had a few of them over the years, but I think I appreciate it more now. What about Harvey's beers? I would love to try them...But perhaps Belgium has the highest concentration of traditionally brewed beers...Any thoughts?
Worthington's White Shield supposedly the same since 1829. However with challenger and northdown hops, both developed in the 70s, this seems unlikely. I wondered about Bass but can't find any info. Pedigree still uses the union system and the same water well but again can't find much on the recipe.

Do we really think the 'same' hops are used? Belgian beers are made with older hops and I suspect a fair mix of varieties goes into them.
I wasn't around in 1829 but White Shield does not taste the same now as it did 20 years ago. Its been around a few different breweries so I would have expected the recipe to have changed. Of course, a change in the brewing process would produce a different taste even if the same hops and malt were used.

Chimay have claimed that the recipe for their beer is unchanged, but they use hop extract, as do quite a lot of Belgian Brewers now. Surely this was not being used a long time ago.

I do know for certain that Greene King IPA is using exactly the same recipe as they used in 1650
I've read accounts of White Shield that have it as a very sediment heavy beer in the past, while modern bottles have a thin, barely visible layer...This alone suggests it was quite different in taste a few decades ago...But who knows? Perhaps it tastes the same, but has the benefit of modern technology. From what I've read Saaz and Styrian Golding hops are the two hops used (there may be another) in the dubbel...I suspect it's very difficult to reproduce beers from before the 20th century...Perhaps the oldest we have date from the fifties, although I suspect Orval comes from the thirties. I wrote this before Alan's reply.
I wonder if Saaz and Styrian Golding hops (see above) are the most common hops used in (traditional) Belgian beer. Even Duvel (a beer that's been around for a long time) has them (and I bet this beer hasn't changed at all since it went pale). Both hops have been in existence for a long time, though Saaz are practically ancient. Anyway, I'm becoming convinced that Belgium is where to go for tradition. I'm sipping an Abbaye des Rocs Brune (see below) that isn't that old, but reeks to me of tradition: thick, sweet (though with some sharpness), pungent, heavy sediment. Brewed with three hops, though I can't find out what they are, but I have no doubt that one is Saaz. It seems to lack the polish of the modern White Shield, for example...which is what you would expect. And the brewery has a dodgy website, which I find strangely admirable!
Hiding behind the sofa, I have never found White Shield to my liking. Extremely fizzy and bland. My first bottle was around 40 years ago, so it has nothing to do with brewery changes!
I used to like White Shield but a bottle bought and drunk this evening is nothing like what I remember.Perfectly drinkable but very dull..