I found one on my doormat on coming home from work this week. It's actually the first tangible evidence that any JW has been at my home for 3 years.
The invitation is really vague - no mention whatsoever of JWs other than a small refence to JW.Org ( but the fact that the tract is published by the WTBTS I guess gives it away. ) Even the ink stamped on KH address doesn't say Jehovah's Witnesses - just "Kingdom Hall".
Seeing the ( slightly offset ) Kingdom Hall address brought back memories of stamping thousands of Kingdom News tracts with the Kingdom Hall address pre-1975 when I was a child. As others have said - there did seem to be some purpose - a sense of finality - back then.