Anyone know how to tell between genuine Manx frame and replica?

I'm going to look at an ex race bike for sale in a couple of weeks time
It had a manx Norton engine fitted but now has a 650 twin
The seller doesn't know for sure if the frame is original or replica
He says it's been bronze welded and has a chassis number
It has the top tube flattened for the manx engine
Anyone know what to look out for ?
Cheers

My understanding is that Manx frames had an oil carrying left hand top tube, this had a fitting towards the rear loop to allow a tap to be screwed in and a fitting (towards the front IIRC) to allow oil to be filled.

This was to lube the primary chain.

New replica frames don’t have this facility as most run belts and anyway, dripping oil onto tracks is generally frowned upon.

I didn’t think honk Manx frames had flattened top tubes ? But could be wrong.

Don’t take someone’s word that it’s bronze welded. People kid themselves.

Fast Eddie said:

My understanding is that Manx frames had an oil carrying left hand top tube, this had a fitting towards the rear loop to allow a tap to be screwed in and a fitting (towards the front IIRC) to allow oil to be filled.

This was to lube the primary chain.

New replica frames don’t have this facility as most run belts and dripping oil onto tracks is generally frowned upon.

I didn’t think honk Manx frames had flattened top tubes ? But could be wrong.

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I've only spoken on the phone so I don't really have too many details
He definitely said a Manx engine came out of it but clearly that doesn't mean it's a Manx chassis

Well the frame number should include

10M2 Manx 40 Featherbed

11M2 Manx 30 Featherbed

If it does then the next question is are the stamps genuine factory.