Hallo,
I saw these postal marks and did some research on it and found that these are experimental postmark. I found a site explaining this and it seems that some types exist. (London two offices only)
Here are two postcard to Bavaria from 1882 and 1886 with these Handtsamps (Two types) one without triangle inside four corners. Some reading below. So another variety to my Bavarian collection.
London fancy geometric postmarks experiment began in 1880 (linns.com)
Best regards
Sylvain