How to categorize these?

These are tricky to me.

They have one date on the obverse, another date on the reverse and various signatures on the back (sometimes with no change in date).

This is one of mine:

Obverse date is 3 May 1979

Reverse date is 20 June 1979

And that one is series VI

Are we going to have various lines as per the date on the back?

Because as it stands we have one line with “1979” in the catalogue, but there is another line with the specific date of 16 August 1979.

Interesting.

I checked scwpm on this. It lists the dates on the front - in the case of your note, 3.5.79. It makes no mention of the date on the reverse. As such, I would go with listing the date on the front.  We might need a more specific catalogue to investigate the significance of the date on the back, which might be associated with the round stamp on the left side. A large bundle of random circulated notes would also be useful to see if there is any variance in the date on the reverse.

I have two other notes:

They aren't too easy to get as UNC, but maybe we can get enough info from sales images

I would take a different approach and record the date on the back. You may have a few pieces of such note all having the same date on the front and different dates and signature on the back, and for identification purposes, I would use the latest date. This is just my opinion.

These seem like overprints, where they put not just the date but a third signature on the back. First time I see that.

The date on the back might be the actual issue date. Can the text in the circular seal on the left be translated - it might give a clue.

This system of dates on the reverse has been in use for quite a while. 

As to combinations of front and back dates, I would suggest listing the front date as the ‘date’, and list the date on the reverse along with the reverse signature - this would enable easy handling of front/reverse combinations should there be any.

These are tricky to me.

They have one date on the obverse, another date on the reverse and various signatures on the back (sometimes with no change in date).

-Yes, El Salvador notes from this era are quite complicated.  You have to pay attention to the Series, the signatures + both front & back dates (& the one signature on the back).

The first note you posted (1979, Series VI, machine code serial # & 1979 on back) looks to be 125b, due to its machine code serial number 7526803. Take this link on the BNM & check how all the serial numbers of P-125a are rounded regular font while there is a transition with P-125b to a machine barcode type of serial number. Your example from Series VI would come before the VN (1st) example shown. The 2nd note you posted is also P-125b (Series ER comes before Series ES on the BNM 3rd/last example). 

I have an example of P-125a (which could be confused as P-133A due to its 1982 date but comes from the 1978 issue, has 3 signatures & is from Series XJ, which comes after XI on the BNM examples). Note the rounded typical serial number font:

Your 3rd note posted (Series GF) appears to be the slightly tougher/last P-133A variety with 2 signatures. 

Remember that there are many series of this 1 Colones issued & that current catalogues may not match all the varieties discovered (one's example might NOT be an exact match to the catalogue's examples). This doesn't necessarily mean that one possesses a “rare variety” because so many millions were produced & the data (from observations) is still incomplete. 

That is great info, thanks. I didn't even notice the change in serial number font!