Unfortunately, my experience with Nimrod was all with current-production (at the time) bottles. I started keeping track of my wine purchases in 2006, and from then until Warre's quit producing Nimrod in mid-2008, I seem to have purchased around 350 bottles (including my 13 case panic purchase when I heard they had ceased production). I have one left, which I am keeping just because I can't face opening it.
Nimrod was a tawny that matched my palate perfectly, and had the further advantage of costing about 1/2 the price of the other tawnies I liked which were mostly 20 YO from various makers. I now drink mostly Ferreira 20 YO, but I'm not sure whether it is really close in flavor/aroma profile to the Nimrod, or whether my tastes changed as I experimented looking for a replacement. Nimrod was an aged tawny, and I heard all kinds of rumors about the actual ages (in barrel) of the wines that were used to blend it. I heard numbers ranging from an average age of 3 years up to 15 years. I have also heard people opine on the age that it "tasted like", and those estimates varied almost as widely.
There was never any indication on the bottles (until very near the end?) of when the wine was put into bottles. I believe I was once told that it was possible to estimate the bottling date from the Selo number, but it was only an estimate because they just grabbed a handful from the unsorted supply of Selos when doing a bottling run. I believe they recorded the Selo numbers with a video of the bottling line, but that seems like a lot of video to store permanently.
Nimrod was (like all tawnies - indeed all Ports) extremely subject to change after opening. It held up fairly well for a couple of weeks and then faded. It never got to tasting bad, but it seemed to become a bit uninteresting after several weeks. But not always. I have a friend who is essentially a non-drinker. He will have a sip of something sweet occasionally, so I gave him a bottle of Nimrod every New Years Day. One year he offered to open it on the day, but recollected that he had an open bottle. Some investigation at the back of the high kitchen cupboard revealed that he had an opened bottle AND an unopened bottle. The opened bottle was likely to have been open for two years and just sitting half-full under the T-cork at room temperature for all that time. It had probably been opened a half dozen times (refreshing the oxygen) to get an occasional sip. It was really good! It had the nutty, slightly astringent flavor that vintage ports can (but don't always) get after 4 to 8 decades of careful aging.
The fill levels seem fairly close (suggesting a similar age). Usually Warre's fill to up inside the capsule, so there has been quite a bit of evaporation on both bottles. Could the Nimrod label have been printed with an "aged" look? I see the etching of Nimrod the Mighty Hunter just as it was in later production bottles.
As you can see, the label is printed on a tan or beige paper, and might get fairly old-looking in a fairly short time (as in not too many decades).
If those were my bottles, I would open them, prepared to drink immediately (in case the remaining flavor and aroma should prove to be fugitive), but also prepared to wait (in case the aroma and flavor should prove to be unpleasant and I wanted to wait to see whether they would improve with a bit of time).
Whether they drink well or not the emptied bottles would make just about as good souvenirs (for me) as the full ones (but probably nowhere near as commercially valuable, but I don't get the impression that you bought these as investments). My own feeling is that wine that I do not intend to drink at some point is worthless to me. An empty bottle (from which I tasted the wine) is just as valuable to me as a full bottle from which I will never drink the wine.
Let us know what you decide, and (if you drink them) how they taste.
