No mystery... it's the shortened serial number that Chevy started using in '57. This is a case where the I was used in place of a one - very common in the engine serial number stampings - so it's F1205F, if I'm reading correctly. The prefix did not use two letters. That translates to Flint, MI Manufacturing plant, 12th month, 5th day, and F is the code for a 283 that originally was used in '57 Passenger car service with a Powerglide.
Here's a couple photos from the '58 Chevy Master Parts Catalog, pp. 12-13 for the decoding. [Edit: added a better photo of the codes on p. 13, showing the "F" suffix.]
Of interest (to me at least), the F code was reused by Chevy, starting in '58 for the 348 "W" engine, so that narrows down the year to '57. There will also be a block casting number and date code, both in raised numbers above the bell housing flange, which can be used to verify.
