Man, the last splicing block I bought was from 1981. Haven't worn it out, though it's got a lot of miles on it. Was always careful about holding the blade vertical so as not to widen out the slot... but it did get deeper over time. I was going to post a link to where it came from, but there are no markings on the block, and the supplier is long gone.
Reminder to demag those razor blades, folks...