This is what I'm not so sure about, since I remember travelling on slam door stock on the services which were first stop Romford for quite a few years after the Southends had gone over to 321 operation. I'm not entirely sure but I believe the fast trains calling at Romford back then went to either Braintree or Colchester Town. In early 1994 I moved away from the area but still visited at least once a year, spring/summer of that year was the last time I remember travelling on slam door stock though, from my next visit onwards (which was in spring 1995), I only ever remember 321s.
I notice some reference on wikipedia to 11 321/4s being transferred to the GE although I can't find any dates for this.
This may well explain it if the 317s released by the 365s around 1995 allowed 321s to be displaced from West Anglia onto the Great Eastern allowing the removal of the last slam door stock from services which stopped at Romford.