That doesn't look as if it had been re-proofed after someone "bubbaed" the re-chambering and did a bad job of overstamping the original .222 marking to make it look like .223. Otherwise I think that the proof house should have struck out the original markings and put on new ones.
At the very least the proof house should have rejected it if the gauging checks failed, as I suspect they would have.
FWIW CIP max. pressure for .222 is 3700 BAR. .223 is 4300 BAR (the same as for Nato EPVAT testing of 5.56, so crack on with mil surplus ammo in confidence.)
Whereas SAAMI (who are only an advisory body), set .223 Remington at only 3800 BAR, albeit using a much cruder method of testing which may not have much read-across. Besides, in the US it seems anyone can make anything and sell it untested.
For your elucidation, see
http://www.gunproof.com/Proof_Memoranda/RULESOFP.PDF
Those proof marks suggest to me that it originally passed, as a .222, in 1996 at the London proof house. AFAIK both UK proof houses keep permanent records of all rifles by make and serial number, so you should be able to ascertain if it was ever sent for re-proof after the conversion, as it was legally required to be.
Selected extracts below:
6.—If a barrel of one sort is converted into a barrel of another sort, (for example a short barrel into a longer barrel or a barrel into a sound moderated barrel or from a fixed choke to an interchangeable choke,) the barrel shall be deemed out of proof from the time when the conversion is begun. (See sections 115 and 130 of the Principal Act, as amended.) Barrels converted 7.— Unless required otherwise by the Sender an arm submitted for re-proof shall, if in the opinion of the Proof Master it ought so to be proved, be proved by such corresponding Definitive Proof or Special Definitive Proof, as that by which it was proved on the last previous occasion of proof or re-proof. If, either at the request of the Sender or option of the Proof Master, it is not so re-proved, it shall be proved as if it had never previously been proved.
9.— (1) The Sender shall deliver each arm and each barrel in a proper state for proof. Condition generally (2) Each barrel shall be fine-bored and turned or ground. (3) Each barrel shall be, if sent for proof, clean, free from rust, pitting, dents and bulges, both internally and externally, or, if sent for re-proof, in a reasonable condition, to the satisfaction of the Proof Master. (4) If an action be fitted to a barrel, it must be tight on the breech face, and, the head space shall be to the satisfaction of the Proof Master.
12.— As to any barrel or arm submitted for Definitive Proof, or Definitive Re-proof, or Special Definitive Proof, or Special Definitive Reproof:— (a) In condition, each barrel shall be struck up, smoothed, and chambered, and, if intended for a rifled arm, rifled. Each barrel shall be fitted to its action, which shall be finished or in the finished state and intended for such arm ...
18.— Before firing a load for any Definitive Proof or Re-proof, or Special Definitive Proof or Re-proof: — Measurement of bore and chamber dimensions. ...
51. — If a barrel or arm already proved and marked as proved shall fail any subsequent proof or re-proof, the Proof Master shall, before returning the whole or any part of it to the Sender, either delete or bar out all existing marks, (excepting any unique identifier or serial number) or ensure, to his satisfaction, that the whole of such barrel or arm will, within a reasonable time, be re-submitted for proof or re-proof’.
