Was intrigued by the title, in the index of posts it was 'Sweat spot' but now I see that it's 'sweet spot' in the post itself.
I don't think there's a particular year of manufacture that stands out when you look at the Standards from the inter war years. If you find a good to great condition one that's been well looked after you will have a very solid well made & potentially pretty accurate air rifle. There may be slight variations between one rifle & another but I don't think the variations encountered either increase/ decrease in frequency or degree by age. It's just the variations you might expect to see in a well controlled manufacturing industry that is making a well designed product with good quality materials by a well trained & experienced workforce.
You kind of know they are good because they are mostly still doing what they were designed to do all these years later.