An experimental piece from later - given the square shape.

I don't know why there are some red swirly bits. -Red glass was turned
into brown glass via the reaction it has with silver salts which were added to the hot glass.
If there is red alone, it menas the silver salts were not used, but it is still basically the "Tortoiseshell" pattern.
Perhaps they ran out of the salts at some points in time? Or simply didn't have enough time to add them.
Most pieces have both streaky reds and browns, from the reaction being incomplete - but it does give rise to a beautiful effect.
There was a later range, in the '80s, of swirly reds with blue/teal strapping - often described as orange and green. The red is rather a burnt, and slightly old-blood-y sort of shade.