Need help on machining issue -- attaching enlarging lens to board

Since I'm clueless in machining matters, I'm stuck again with mismatched parts and looking for assistance.

I have purchased an El-Nikkor 150, which is threaded on the back -- 39mm, I'm told. Looking at KBX Photgraphix for a disc to mount it in, to hold it to the cone of my D2 (I have one for my 135 Componon, held by retaining ring, but much smaller than the Nikkor back), I had a choice of an out-of stock used disk with the 39mm threaded flange already included or an in-stock plain disk with a 39mm hole. Knowing that the former could be years away, I bought the latter, albeit at an ouch-price with the shipping, and gladly took the offer of a flange at a great price from a member here.

The problem is, the hole in the disk is just slightly too small to accommodate the threaded part of the flange. The screw holes in the flange are for countersunk heads, but of course that side is on the side opposite the threaded part. Therefore, I need to either

1. pay a machinist to widen the hole and tap or drill the three, #3-56 screw holes into the board to attach the flange (though I am concerned that widening the hole would get it very close to where the 3 flange screw holes would be. The disk is 1.5mm thick. KBX says it's plenty thick enough to thread); or

2. reverse the flange, putting the countersunk side against the disk, and use flat head screws. Then the question of clearance from the top of the heads to the back of the screwed in lens may be an issue; or

3. find a retaining ring for the back of the Nikkor. I have no idea what the thread is.

Given what I've already spent, I will great grateful for learned advice.

I hope the photos help.

Attachment 240321Attachment 240322Attachment 240323

Lens1 and Lens 2 show the the non-countersink side of the flange. Lens3 shows the potential head-clearance issue. The distance with the lens fully screwed in appears to be about 1/16".

Buy an m39 ring for little money.

It's probably M39x1, and not M39x0.75.

Buy them both if in doubt. Lot of version on ebay.

Not sure why you need to screw the flange to the board or fit the threaded part through the disk opening. If the lens fits through the hole, just screw the flange to it, flat part facing the mounting plate. Something like this:

Attachment 240334

if it's a Omega part number 421-013 you are looking for, I have two.
Yes Postage from Canada is pesky.
Bill

I thank you all.

Pieter wins the prize for the solution. It works perfectly. Why didn't I think of it? Because I am terrible at anything to do with putting things together. (Can you tell?) Thank you, Pieter; remind me to give you a raise next year.

And thank you, cowanw, for your offer. But since I have what I need, I'm all set and can print tonight.

As mentioned, you can use the flange as a retaining ring, as I have done here:

Attachment 240365