The calculator can easily be opened by removing three screws.

The picture on the left shows the main board of the Omron 12SR calculator. The big chip together with the two smaller chips below is a three chipset calculator from MOSTEK.
The three identical lined up chips in the middle are vacuum fluorescent display drivers. The discrete components above these display drivers are a buck converter to generate the high voltage needed for the fluorescent display tube.

On the picture below the IC type numbers are readable.

Calculator chipset:

MOSTEK  MK 50075N   Data Processor
MOSTEK  MK 50103N   ROM 1
MOSTEK  MK 50104N   ROM 2

VFD display drivers:

Toshiba TM4358P-2