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Hi Hans,
That's a 20mm Type 2 Nagler, there was no 20mm Type 1 Nagler.
The Type 1 Naglers were:-
4.8mm, 7mm, 9mm, 11mm and 13mm.
The Type 2 Naglers released several years after the Type 1's, were:-
12mm, 16mm and 20mm
The Type 2 Naglers significantly alleviated the Kidney Beaning and Blackout issues associated with the Type 1 Naglers.
While it's an absolute housebrick, the 20mm T2 Nagler is still a great eyepiece optically.
Cheers
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