This is an older Heros block set, # 4/20, with 41 pieces. The 4 probably means there are even smaller sets, with fewer pieces. Or maybe it means "about 4x10 pieces." The 20 probably refers to the 20mm block pitch (a little over 3/4").
The sheet with sample designs is taped inside the box lid. Unfortunately, several of the designs are for larger sets, or two sets, since they show four arches, instead of the two included.
The blocks are mostly 20mm x 20mm by some multiple of 20mm, with a nice bias towards longer blocks. Some pieces are half thickness of 10mm, and the arches are 30mm, so there can be some matching necessary for heights to come out.
The colors are bright, the surfaces slick, with typical tumble-finished characteristics, including a bit of dimensional unevenness and occasional slight lumpiness. Those irregularities can make for some problems in construction, but I am so fond of tumble-finished wooden pieces, that I don't mind.
Heros current production may be more dimensionally consistent. I'll need to find a store that carries some.