Hello Dave.
Thank you for expending so much energy on this. It remains something of a mystery, doesn't it? There's a problem in hoping that it was sold to Woodham's and then sold on to another scrap yard as 4222. The available "high level" evidence from BRDatabase suggests it was withdrawn from Llantrisant on 9th October 1964 and scrapped during February 1965 although it says not where. Stephenson Loco Society gives withdrawal as 12th October 1964, so not a million light years away. Rail UK says the last shed was Pontypool Road from where it was withdrawn on 31st October 1964 which may well be a "period end" date rather than the actual date of withdrawal. However it gives a scrapping date at Steel Supply Co Jersey Marine (near Pontypool) of 28th February 1965, the date confirming that on BRDatabase.
If those dates are correct I couldn't possibly have seen and recorded 4222 at Woodham's in November 1967.
I have no knowledge of whether or not it was possibly for rods to be exchanged between locos on the WR. It certainly happened on the SR as, when Steph and I went to photograph the T9 120 in pieces at the Flour Mill an inspection of the internal valve gear showed parts had been on an M7 previously - they were stamped with the full number. My memory tells me that the same applied to GWR locos. So, is it possible that the rods from 4222 were transferred to another loco when it was withdrawn? At that time record keeping was becoming very "relaxed" and in the knowledge that the transferee was likely to be withdrawn soon the exchange may not have been properly stamped.
This, then, brings us to the next problem, as the loco I recorded as 4222 is not, by all appearances and your research, one of the 42/52XXs known to have ever been at Barry. We therefore come full circle to the possibility that 4222 at Barry is not actually 4222 but another of the class which was sold on to another scrap yard and never recorded at Barry despite the number of people who visited the yard in the '60s and '70s.
Unless someone knows differently.........
Brian