SAE/AISI 4138 is a 1.1% Cr, 0.25% Mo alloy, from the same family as the 4130 we're all used to in low to mid-range steel tubesets.
The first two digits in the AISI nomenclature refer to the alloy composition family, in this case the 4100 series, 1.1% Cr, 0.25% Mo, and the last two digits refer to the average carbon content in hundredths of a percent. In 4130, that's 0.30% C, and in reality usually falls between .28% and 0.32%. In 4138, that's 0.38% C, nominally identical to the better known 4140 alloy.

I'm not calling b*llsh*t one way or the other on the actuality of these forks being 4138, merely informing you of how the naming system works :-)