The P38 came with two types of bumpers. One had a smooth finish like of the Westminster and H&H and a few other special editions, and the other was a type of Matt rougher type finish which is common on most P38s.
The other morning I came out of shops and found someone had hit my rear bumper hard enough to split it almost in half, beyond repair. I can get a bumper in the Matt type finish at local scrap yard for $100 but before I do I wanted to ask if you guys know whether it’s possible to have a paint/body shop transform the bumpers finish to a smooth one to match the front bumpers finish or will I have to search for a smooth finish bumper? Local boys shop says no problem, but I’m sceptical...

They should be able to build it up with putty primer, might take a few coats, but certainly possible

Although it's plastic as long as you don't get carried away and get it too hot, it should be possible to sand it smooth.