While hunting for a cheap R-R opamp that's readily available here in India, I came across the MCP6001 and 6002. Unusually for the Indian market, a few sites offer several variants of both single and dual types.
The first three suffix letters stand for temperature range, package and packing but some have the four extra letters XBLW. These variants for both single and dual types cost less than a third of the others (roughly US$0.20 vs $0.75) but I cannot find what the letters stand for. I looked at datasheets from various sources including the latest one from Microchip but couldn't find any reference to this extra suffix.
I assume that the extra letters are not batch or date codes. Chinese copies maybe??