When the lyophilized stock material of a product is running low, a new batch (10 - 50 units depending on the frequency of sale) is lyophilized. One unit of each new batch is taken for quality control testing and is stocked for sale only after it has passed all quality control tests.
The new lot is available for sale once the previous lot is sold out. Due to this schedule, all products are always on stock and ready for sale.
Hence, it is not possible to choose among different lots. Buying the same lot again is only possible if that lot is not yet out of stock.