The door of George's (Jason Alexander) new apartment first shows '608.' When the head if the tenant association visits him later she mentions Mr Eldridge from apartment 8c. Apartment buildings do not use two ways to number their apartments.
Kramer tells George that 51 people died and 1660 were rescued in the Andrea Doria/Stockholm collision. The total dead indeed is the combined total that died: 46 on the Andrea Doria and 6 on the Stockholm. However the total stated for survivors, 1660, is the total only for the Andrea Doria. The Stockholm had 750 survivors.
The post office has no way to know what percentage of people get their mail unless the people complain and anything under 100% wouldn't be acceptable.
In the prologue, there is no hot dog in the hot dog bun Kramer stuffs in his mouth. Most likely to prevent a choking hazard.
George (Jason Alexander) is supposed to be 34 in this episode, but he gave his age as 33 in The Heart Attack (1991), six years earlier.