The bigger digits underneath would be the original numbers.
The fist 6 digits are production numbers.
Meaning how many of that type of engine was built at the Flint, Michigan engine assembly plant.
The small digits on top of the original numbers are not something you would see on a GM replacement engine.
I was told that when a replacement engine was issued,
GM would send the mechanic a stamping die set to transfer the original engine serial numbers to the new replacement engine.
When done, the mechanic would send the stamping die set back to GM Headquarters.
In some states, DMV would use the engine serial numbers on the vehicle title, some used the vehicle serial number on the title.
You should check your title to see what number was used.
What ever is on the title it should match ether the ESN or VSN
F56TA - looks like original stamping
F - engine assembly plant, Flint, Michigan
56 - model year, 1956
TA- series & type designation, 6000 - RPO 225 HD heavy duty engine 261 cubic inch
You should check your other
Block NUMBERS to verify what you have, you probably have 261 engine.
And to check block casting date.
Check here for 1956 Resto pack 
