Let me try to answer a bit...
First of all, are you familiar with this?
https://www.sl113.org/wiki/Electrical/TailLightsYou are a Full Member, you have full access.
This is also not a bad source of information:
https://www.sls-hh-shop.de/main/de/mercedes-230-280sl-w113/82-beleuchtung-scheibenwischer/82-i-info-rueckleuchtenI left it in German for a reason. It is easy to translate it via Google and in German there is far more information than in English.
I do not suppose that the part numbers will be helpful while buying on ebay. But anyway - to confirm fully your part numbers, your VIN is required as the part numbers work by VIN, not by year.
Part numbers you provided:
A1138260152 - it is the pert number just for lense (cover), not for lamp; yes, it is for the US, red blinker, left, UP TO CHASSIS 011040, replaced with two other part numbers, last one being the whole lamp (but I cannot tell if these replacements were all red)
A 1138260256 - this one is also lens, right hand side, but AS OF CHASSIS 011041 - so from a different set up
The right hand side to match your left hand side number would be A 113 826 02 52.
So first determination is:
1. Is your car US? You can find it:
first go there:
https://www.sl113.org/wiki/DataCard/BodyandPaintNumberPlatethen there:
https://www.sl113.org/wiki/DataCard/DealerandCountryCodes2. The VIN number - is it above or below 011041?