Anyone tell me what they mean or are they inconsequential???
Also what your take on what looks to be a crack??? Block and crank are off to machine shop next week. (Assuming I can get front hub off) so not too worried about it, but what would have caused it???

First set of numbers
Next set of numbers

A \"b"

The crack right side of left relief

 

Looks like the markings of a standard size piston. Bored block pistons will have numbers like 010; 020; 030 indicating that the block was bored to that size.
If the crack is not superficial, I would replace it.
When I did my 350, I ordered all of my engine parts from Summitt Racing.
Since yours is going to a machine shop, they may be able to locate parts cheaper.

 

I think the "crack" looks odd. It looks more like just a scratch because it ends so abruptly and cleanly at each end. I don't see the "crack" growing into the valve relief, as one would normally. Do you see anything on the under-side of the piston top? See what the machine shop says. I believe the numbers in your 1st photo are related to the manufacturing date of the piston: 11(hr), 21(day), 08(month), 93(year). Also, check the last digits for the OEM replacement piston from Chevrolet. Do they coincide with any of the numbers?

 

I suspect that these pistons are just fine for reuse. sourced from a low mileage engine and from a source that had scheduled maintenance.

OTH the machinist can measure and validate them or finds them lacking. Still within "spec, or worn..Diameters. ring land grooves?

Caveat, the pistons must face properly. The relief's match the valve travel, When cold may not "touch"., but when warm the "kiss of death" valve to piston !!! Not me, but close relative!!

Carl