I've noticed the likes this is getting, and I found the answer myself. It follows:
T52-4 was the final head to head blend that was competing internally with T52-3 to become the T52 final blend. T52-4 was exceptional, in fact JD and I both liked it a slight bit better, but we were concerned that it tended to overpower some of the nuances in the liga and that we were sacrificing flavors for strength, so we ultimately decided that the T52-3 would be the final T52 blend.
JD for his own smoking pleasure kept having T52-4s made, but we banded them as JD4 so as to not cause confusion in the factory or with those he shared them with. After he started handing them out, he decided that he didn't like putting his personal name on the product, so we started ringing them with MF-4 - it stood for what you think it does. In the factory we kept tinkering with the T52-4/JD4/MF-4 blend to try to improve it (in our opinion) - goal was to keep the octane, but restore the depth of flavors... so we made a variety of minor liga tweaks and vitola changes to try and get it "perfect".
The result was the MF-13 - a robusto format. I think the MF-13 is better, not sure where JD stands on this - truth is the difference is so very minor between the two cigars I doubt most folks could even tell there was any difference unless they smoked them side by side... and maybe not even then, but we can tell... or so we tell ourselves. ;>
We started sharing both MF-4s and MF-13s at events and out of our pockets and they started to get some buzz online.
JD came to me one day with a My Father cigar and pointed out that Pepin's rings have an MF in the center of them. How both of us missed this is kinda crazy since we both smoke a lot of their cigars, but we did. So we decided to change the name to UF out of respect - we take pride in not knowingly copying others, so we rebanded all the MFs as UFs on our own accord.
UF stands for "Unico Fuerte."
So MF-4 = UF-4 and MF-13 = UF-13. Netiher blend is really in testing per se, both are finalized blends. It just that neither is being produced for retail at this point. We would like to release the cigar this year, but can't commit to doing so until we are confident we can get T52, Dirty Rats and L40s delivered on a regular basis to our customers as the UF-13 shares the same Stalk Cut Habano capa.
Now you know more than 99% of DE does about the lineage of UF-13... ;>
This was posted by Steve Saka on another forum.