To properly configure the Macintosh SE 2/40 (or any Macintosh SE with a
RAM configuration jumper labeled "2/4MB to 1MB") for 2.5MB, the jumper
must be completely removed.
Apple suggests leaving the jumper connected to one pin so that it is
available for future upgrades.
NOTE: The RAM configuration for the Macintosh SE 2/40 is the OPPOSITE of
previous Macintosh SE and Macintosh Plus systems. The highest capacity
SIMMs occupy sockets 3 and 4 on the newer boards with the jumper RAM
configuration. Older boards with the solder configuration require the
highest capacity SIMMs to occupy sockets 1 and 2.