What is this on my 1948 penny...?

Ugly...Almost looks like some one welded across the penny so to speak...!
Would be interesting if it is in any books as an error/variety...

Wert, when a coin die develops a crack, it spreads under pressure when it hits the face of the coin. This is what you are seeing. Your comment about it looking like a weld is interesting because superficially it does look like that. The spot where the die crack is displayed on your coin is a primary area where I'd expect one to develop, from a denticle to a design element (bust in this case).

This era of coins features many variations and many were catalogued by a man named Hans Zoell and you can buy a Charlton-Zoell variety catalogue.

Considering your interest in finding and examining these types of coins, I recommend that you get one.

Its a die crack, commonly referred to as a neck snake and its fairly common on the pennies from 37-52 l have several years showing this

Wert: I had the same issue a short time back... and this was a response from a community member:
Quote from pgrinh:
Your bottom picture is Zoell variety number L72d. There are three total listed 1948 one cent Zoell varieties Bust to rim, none of which match your top picture..... This is a particularly prone area to Die Cracks for all denominations of the George VI coins.
Hope this helps.

I see that Dvereckis ....Yours looks like what I thought a crack should look like...Mine looks more like some one welded the penny together (if you know what I mean).
Nice coin you have there....I will have to keep my eyes open for cracks in coins from the mid 40's to mid 50's.

Thats nothing compared to a 1964 penny I have. I'm not sure if the long cracks and missing chips might make it a shattered die though.