David- very glad to see your TN, and also very grateful of your account of the bottle’s ullage etc.
First off- to take on the SOM, I use it for wines young and old and for vintages that are either old or potentially weak/problematic I think it is essential. I make a habit of tasting off-vintage Bordeaux (as in 1972-1974, 1965 etc.) when I find such bottles- and I do not even decant. I just pop and pour and keep close watch, sometimes for hours, until the wine has peaked and faded.
As for 1975 Chateau Margaux, I note with great interest your bottle has a low neck fill- and I wonder if that might also contribute to the positive showing you experienced relative to what many tasters report when trying this wine.
The 1970s for both Lafite and Margaux are a particular point of contention when talking about wine critique- especially Broadbent vs. Parker- and so I have focused on those wines from that era more than I might otherwise.
I can see where these two gentlemen differ on Lafite- and I personally do not care for the 1970, think the 1971 is perfectly lovely if slowly aired for at least an hour, and am a fan of 1975 and 1976. Generally speaking, I have found bottles to be sound with the difference of opinion being about what is inside the bottle.
Not so with Chateau Margaux. The 1975 in particular. In my appraisal work, I have come across 1975 Margaux many, many times. And all too often the bottles have failing corks and low to mid shoulder fills or worse. Looking back, I would say that on average 1975 Margaux I have examined have a mid shoulder fill. This is not exactly a complete examination of the entire population of extant bottles, but it is uniquely problematic in my overall experience. I can think of no other wine at the first growth level where I have seen so many examples with poor ullage and signs of leakage. It goes beyond possible importing issues.
I have a theory about this. We know that the former owners of Margaux were facing financial difficulties in the years leading up to the sale of the estate in the late 1970s. My own tasting experience also suggests the quality of the wines came up dramatically once taken over by the Mentzelopolous family- a sudden change that could not be attributed to better care in the vineyard alone (though that did come fully to be in the 1980s.) Additionally, once you get into the Mentzelopolous era, the issues with ullage and leaky corks stop.
As poorly made as the wine surely was at the time, I think cheap corks may have played a role in how 1975 (and the 1970 to a lesser extent) perform at the dinner table these days. The ullage rates and cork failure on bottles I have appraised is akin to very inexpensive crus bourgeois of the same era. All of my tasting experiences with this wine suggest very tired bottles- something that should not happen with a sturdy vintage like 1975. Sure, many 1975s are not that great- but tired and oxidised should not be a common problem just yet at the first growth level. 1975 Margaux is not a great wine, but I would sure love to try a few examples that have the ullage and cork condition one might expect.
Anyhow, yours is the first TN I have ever read for this wine on a wine forum where someone indicated they opened a bottle with acceptable ullage and had it come out pleasant to drink. And so I thank you. And if the rest of your bottles tend to have similar ullage, it may well be you lucked into a batch bottled with corks that have held up much better than is often the case.