What is this objective adapter?

[Sure Squintsalot]

I picked up an old Nikon M Plan 40 objective and it had this adapter on it:

Screenshot 2022-08-19 194612.jpg (25.96 KiB) Viewed 994 times

Anyone know what it is? The identical knurling to the objective makes me think it's a Nikon part. The outer diameter of the adapter threads is 26.0mm:

Screenshot 2022-08-19 194554.jpg (48.19 KiB) Viewed 994 times

[Greg Howald]

Looks to me like an adaptor ring used to make a 20 mm objective fit a 25 mm nose piece. I'd like to find a source for such things at a reasonable price.

[PeteM]

Looks to me like an adaptor ring used to make a 20 mm objective fit a 25 mm nose piece. I'd like to find a source for such things at a reasonable price.

Greg - eBay is one source: https://www.ebay.com/sch/i.html?_from=R ... =2&_sop=15

[blekenbleu]

I have accumulated a couple of them and suppose that they are from Nikon
specifically for using 210 M Plan objectives parfocally in BD Plan nosepieces.

M26-RMS.jpg (32.07 KiB) Viewed 913 times

[Greg Howald]

Thanks Pete.