Yaba pills contain Burmese meth produced in the golden triangle right? After all, it's the main Asian supplier and it's pretty close to the main yaba markets which are mainly Thailand and Cambodia if I'm not mistaken (maybe some other neighboring countries as well).
There isn't a lot of reliable info about it, but according to one review from Singapore, the vast majority of the meth seized in Malaysia & Singapore is dextromethamphetamine. However, most seizures are from crystal meth/ice which is obviously a much higher quality product.
So I'm not sure but my educated guess is that yaba contains a small amount of d-meth (probably less than 40-35% ) and the rest is caffeine and cuts/binders.
But since they're so cheap it's not impossible that some of them could contain undesirable l-meth or even a mix of products from synthesis that went wrong (or something like that).