RENE has some info I think. Maybe something about them being in 243 and 308. Think RENE also mentioned some "U" but I don't have it handy at the moment.
I'm just not that big of a rifle guy ..... you remember if this one is a prefix serial?
Model 37117 SR#D712-23852 catalog # HM77R caliber 223REM satin blue read that right off the end of the box it came in. :wink:
712 perfix looks to be a 2015 hawkeye
There are "D" guns out there. Quantities will never be known however.
I suppose most of the people who have a "D" marked Ruger don't recognize the significance of the "D".
Chet15