I comparing the 5103 to a 5224 with out disassembling:
The 51 is smaller than 52
The 51 appears to be bushing crank.
The 52 appears to be twin ball bearings.
The finish on both engines is similar with the in box 51 being slightly Grayer.

Also for finish comparison is pic of sand cast Fox 1949 ECJ 6 .35 stunt with a Forster .29 that just guessing would think it is sand cast.

For same is pic of of what we on the Topside or North end of the ball call a Bath Tub intake.
Sand cast Fox .29R " The Worlds Most Advanced Racing Motor" Duke was funny that way!

I remember breaking in ENYA lapped engines, Very painful for the fingers. Hooking them to a quart can and just let them run.
Last forever once broke in.