A good question, and one that is not understood by our Police Firearms department.
I was looking to buy a proof marked complete barreled action (with bolt) and the Police told me that once I had replaced the 'furniture' I would have to submit it for re-proofing - they still insisted even after I explained that their statement was bovine excrement, it finally took a letter from the proof house explaining that they made the proof tests WITHOUT the furniture (due to potential damage whilst in the clamps), and stated that it did say on their web-site that furniture should be removed before the rifle is submitted for testing - that the Police 'caved-in'.