Where did these small tan bricks come from?
These look like Modulex bricks:


Modulex was a building system that was designed for adults. It was developed by The LEGO Group at the same time as their Modulex sets were produced from 1963 until 1965, and were exclusively available in parts packs by colour. The LEGO Group made Modulex a separate company in 1965, and the text &quot;LEGO&quot; was removed from the studs and replaced with &quot;M&quot;.
Modulex bricks were formed with a cube type 5 to 5 width/length ratio, meaning that, unlike very nearly every product produced by The LEGO Group, they were not compatible with LEGO bricks, which have a 6.5 to 5 width/length ratio. Modulex bricks were available in a wide array of unusual colours, shapes, and sizes.
from Brickipedia