Is it possible to build a franken LE including a caseback with LE number?

With your example, the Gemini IV watch, indeed in order to get a new case back for this model the old one has to be sent to the factory first. In fact you can't even buy a new sapphire glass Gemini IV insert for the case back without sending the old one back. But my recollection is that this wasn't always the case as I think I've bought one of these before without sending the old one back (it was for a genuine Gemini IV watch).

This would be true of the numbered parts of any LE watch. But if the LE is not numbered, then all bets are off. The most common one is the Mitsukoshi Speedmaster - not numbered and there are probably more converted watches out there than genuine models.

Cheers, Al

Yea any LE really need the papers or send it to omega for immediate service to validate it’s an LE and if it is not hope the seller allows a return.

There is no need to send it off for service right away - if you can get the seller to provide you with the full serial number, that's the key thing.

You can then call Omega and provide them with the serial number of the watch, and then ask them to confirm the reference number. If it looks like it's an LE Speedmaster for example, but the reference comes back as a 35705000 (or whatever the new reference number is for a plain Speedy), then you know it's a franken watch.

You can then decide to buy it or not and not have to worry about a return.

I know there are reputable sellers here who do this check before they put the watch up for sale actually...

Cheers, Al