Your grip frame marking of XRN-3 identifies it as the "XR3-RED SIZE", the standard Blackhawk and Single Six grip frame size of post 1962 Old Models and New Models.
Here's my complete list:
All GRIP FRAME Identification Numbers, “Founders Marks”, Marked on GRIP FRAMES (N/A to Bearcat)
BLUE NM:
Small steel (OM #XR3 size) 1/8” closer to trigger guard than XR3-RED size, (cut for indexing pawl system, no extensions into main frame & only 7/16” thick) = A (in a square), no other marking observed; w & w/o lock. New Vaq w/o lock has a 512 prefix serial #
Large steel (OM #XR3-RED size): Early = A (in a square) XR-3 (that's XR dash 3), Late = A & T in separate 1/8” circles
Large alloy (OM #XR3-RED size) w/o lock: Early = NXR3-RED, Late = XRN-3RED. With lock = XR & L
Short steel 'Vaquerito’ .32s = A (in a square) XR-3. If ordered as a separate part = 302 A (in a square) 3/16” shorter in length than the XR3 and XR3-RED sizes
SBH = No marking on Dragoon frames. Large round guard frames (OM #XR3-RED size supplied on all barrel lengths less than 7 ½”) = same as above marking
Birdshead** = # in circle, MR00307, A in circle
STAINLESS NM:
Small, (XR3 size) 1/8” shorter (cut for indexing pawl system, no extensions into main frame & only 7/16” thick = K (in circle); w & w/o lock
Large (OM #XR3-RED size): Earliest = KXR-3*, Next = KXR-3 with a separate N, Late = K (in circle) XRN-3
Short 'Vaquerito’ .32s = KXR-3* 3/16” shorter in length than the XR3 and XR3-RED sizes
SBH Hunter = KHRN 3 (Dragoon length but with a round trigger guard)
SBH = No marking on Dragoon frames. Large round guard frames (OM #XR3-RED size supplied on all barrel lengths less than 7 ½”) = same as ‘Large’ above markings
Birdshead** = # K in circle, MR00307
BLUE OM:
Small alloy = XR3, original style to 1962, 1/8” closer to trigger guard than large size
Large alloy = XR3-RED, (part # SS3), new style in 1962 to 1972, also used on Pre 1985 Old Army alloy w/narrow trig slot, and on Hawkeye alloy w/wide trigger slot
SBH = steel, no marking; Early ‘long frame’ marked with C (only 300 made)
Old Army steel (OM XR3-RED size) = XRN-3 beginning 1985 with wide trigger slot
STAINLESS OM - Old Army only:
Large (OM XR3-RED size) = KXR-3*
BRASS FRAMES OM only:
Super Blackhawk Brass “Dragoon” style with square back trigger guard. Same dimensions as 1959 Super Blackhawk. Marked “MR-3DB.” Initially for Super Blackhawks only, then shipped on various Blackhawks, majority in 1972. The ears on these are too short for post 1997 New Models.
*Note: same # used on two different frames: OM SS Old Army and NM SS XR#-RED size.
**Birdsheads supplied on New Vaqueros cut for free spin pawl system and have no extensions into main frame.