That's a good point. That style of triangular plate was introduced in 1921 with nickel plating. In fact all the Meccano parts were nickel plated at that time.
The first painted (red) version didn't appear until 1926, the start of "The New Meccano" red/green era.
If anyone is interested in the detailed history of various Meccano parts, see:
http://www.nzmeccano.com/Parts.php