Just bumping this thread, as I need someone who could provide some empirical data to support my theory about LL below (I copied below text from a post on fb I made earlier as a response to a similar question):
I am not 100% sure, BUT, the 383 and 426w/Hemi shared the same standard bore size, with the 383 being a Low deck and the 426 engines a Raised deck. On the Raised deck 426 blocks an 'RL' is often cast in at the foundry. It could be to distinguish casting molds between the Low and Raised deck engines sharing the same bore. Also, the 'LL' was already found on the first 383s from the late 50s Coincidentally a Raised version of the 383 [2120329]
was also available in 1959-60 with a small bore of 4.03" and 3.75" stroke. These Raised 383s used casting RS. My guess is that the first letter designates the engine platform Low Deck or Raised Deck. The second letter could designate bore size and to be able to distinguish between molds, either Small (S) or Large (L). Small bore would be the 4.03" and I have only seen it on the RB 383. The large bore would be 4.25", which was the largest bore at the time. However, 413 blocks with casting 2468030 and bore size 4.1875" (in between 4.03" and 4.25") also had RL and it could be that these blocks have the cylinder wall thickness of a 426, but only bored to 4.1875". These blocks would have relatively thicker cylinder walls.
To add to the last item about the 413s; I know that back in the mid 60s some 413s and 426w blocks were cast with only 2468 cast in, but with RL. If 361 block also had LL, the cylinder wall thickness also might allow boring to 4.25", but sonic testing would give a definitive answer.
*Edit* Initially I did not check the combination LS, but apparently the LS was cast on early 361 blocks (#1944959), probably using molds from the smaller bore 350 (#1944929). According to my theory this would correspond to Low deck, Small bore. Compared to the 383 available at the time (late 50s) that is exactly what the 361 was.
When the 440 was introduced, it replaced the 426w passenger car engine as the only RB wedge in the engine lineup and designation for both the Raised deck as well as bore size was no longer required (413s were still used for Industrial applications with a different casting). On the 383 it was also phased out at some point (probably after the 361 passenger car engine was discontinued), as I have seen blocks without LL.
Anyway, what I would like to find out is if the earlier 413 blocks from 1959 onwards had RL cast in or not.
Besides meeting someone who
exactly knows what the LL, RL and RS mean, some empirical data would be needed to (dis)prove my theory. So, if everyone could check their 383, 413 and 426 casting numbers and dates; that would be nice