I'd agree on the serial numbers...My Pioneer has no numbers, but my Cross Fell from a few years earlier has on on the BB shell and on a flat filed into the steerer tube.
Orders would have been coming in very fast for Andy by 1988/89, so I suspect he probably dropped bothering with the numbers in favour of getting them out the door.
Overbury's was just Andy making the frames, and his mum Enid handling the front end and the shop...also, their stock management, record keeping and paperwork could be described as lax at the best of times.
On the colour, the yellow looks spot on, but it might have just been plain with blue stickers...although most Pioneers did have a fade of some sort, there was a small upcharge for it.
The curved and oval chainstays would suggest that it's Columbus tubing, so probably 1989.
I had a June 1988 Pioneer, which was still Reynolds 531, with the s-bend 22.2mm round chainstays.
All the best,