The first is a PK-13 extension tube which works great with all my other lenses but specially with the 180mm 2.8 ED AIS and 105mm 1.8 AIS. I haven't tested it with the 300mm.
The second is, as you can see, a 300mm 4.5 ED-IF AI. This lens is lovely and it was cheap because of stains on the border of two elements. BUT since they're on the border, there's no influence on images. It's a sample from the first series, not AIS. I suspect some previous owner let fungus grow and these stains are a result from the cleaning process.
Both are in VERY good condition, ready to serve for years, I hope.
This lens is charming - small, light, powerful, amazing color rendition, extremely fast focusing. If you use Nikon manual focus and happen to cross with a sample at a nice price, get it. You'll hardly regret it.
I picked it used for around US$ 165,00. Well, it has some stains on two elements' coating, but its only on their borders, not interfering on the image. Only on price ;-) I've just posted an image created with it here and you can check about this. I recommend a frequent visit to eBay and other sites the like. It's really worth and you don't need to win the lottery for one. But for a 300/2, yes! Twice!! Sorry for the delay.
When i see your tele.. im thinking to myself that i need one as well.. ohh.. they are so costly though. 4-6k euros.. ouch. Guess i need to win on the lottery first.. hehe
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