The 4-digit numbers are the serial numbers on "standard" Etruria production models. I don't know whether they are based on numbers per (sub)model of pen, but I did notice that the younger a pen is, manufacturing date wise, the higher the number is.
Similarly, I would think that the numbers consisting of a single alphabetic and 4 digits follow the same pattern.
With regard to LEs and SEs, you'll find that LEs normally have a serial number followed by a slash ("/") followed by the total number of LEs to be produced or having been produced. It seems clear to me from discussions on some of the weird numbers. that the trial runs sometimes are sold as well. Regarding SEs, of which I have a few, sometimes have a 3-digit number only, and sometimes have a similar numbering system as the LEs do. This may be caused by marketing more than anything else; I think that true LEs have a digits/digits pattern for a serial number.
Generally, IOW, a standard serial number is of the form:
#### for Etrurias
or
A#### for the Ventidue, apparently
I have also seen (have one)
###
for earlier production versions (which yoru pen also is)..
So it seems possible that even 2-digit and 1-digit numbers exits.
An LE number is of the form:
##/##
###/###
####/####
The size of the numbers depends on the number produced, and if the production number essentially would fit with less digits than the maximum, zeroes are used to space it out, f.e.,
09/50
040/181
0213/2002
An SE number consists, AFAIK, always of three digits, IOW
###
where zeroes are used to fill out the number if necessary, f.e.
018
Note that SEs often run in very small numbers, potentially less than LEs. Actually, the only LE I know with numbers smaller than the largest SE, is the FPH Blue Dusk, of which only 50 were made, while several of the SEs consisted of 60 and 70 pens. However, quite a few had even smaller production runs.
The largest LE number I am aware of, is that of the Étéré, which has a total LE number of 2002. Its significance is the date it represents, namely 20-02 2002

. The largest LE number I am aware of other than this one, is the 10th Anniversary edition, the hazelnut celluloid one, issued in 2001, with a total LE number of 991.
BTW, the engraving is wrong sometimes, I've noticed. I know of an Alter Ego without a serial number, and I have one pen which is 40/60, but over the 0 in 40 there also is a slightly smaller 1 engraved. Furthermore, I have a 069 SE, of which I know that only 60 were produced, and finally, I have an Etruria Nera (2006) which is numbered 35/88, and that should have been a 4-digit number

.
Anyway, coming back to your pen, the number 326 is not necessarily a high number, because it was in principle a standard production Etruria. It just is a fairly old pen, probably dating from somewhere before 1995, and therefore, if anything, is a very low number.
Warm regards, Wim