Great Britian One Shilling Brown Plate 13

Hi to all
Whilst browsing through the family accumulation I found this.
According to SG Collect British Stamps 2002 eidtion, it is number 151 one shilling brown plate 13 with wmk spray of rose type 33
It fits the bill. Back in 2002 it was quite valuable in used condition it was 350 pounds. However, the whooping huge postmark will detract quite a bit. But is is nice to know that I have a rare classic Great Britain stamp. However, can anyone identify the postmark for me.
Regards
Horamakhet.

Hi, This 173 postmark is from Chatham in Kent.

Nicely preserved. I haven't seen one of these close up before, thanks for sharing! This presents well despite it's flaws. Nice.

Hi nigelc
thanks for the information
Horamakhet