The (Fred) Routledge designed 22 smooth bore was used by Winchester in their M61.
Mr Routledge designed the mini indoor trap game 'Moskeeto' in the late 30's. He also developed his 'Routledge Bore' design for .22LR Shot cartridges that threw better patterns than a standard single dia .22 smooth bore bbl.

(The following specs from Win Slide Action Rifles VolII by Schwing)
A Routledge Bore is the first 10.5" @ .217d, then the remaining 13.5" of the 24"bbl of the M61 is counter bored to .375".
The claim was an 80% pattern in a 6" circle @ 35ft.
The straight thru smooth bored (.217) bbl was said to throw patterns 3 times larger (and more).

Winchester & Routledge entered into some sort of agreement ($$) around 1939 to produce the M61 .22LR Shot with this Routledge bore in it.
Winchester quickly decided to change to it's own version of the 2 stepped bore dia and could stop paying Fred for the use of his design.
Win factory info shows drawings for their own version in 1940.

The Win version was a .217 smooth bore from the breech extending 16" forward, then expanding to a counter bore dia of .265" (instead of the Routledge .375") for the remaining 8" of the 24" M61 BBl.

M61s with original 'Routledge bore bbls are very rare as production was short lived.

The last version of the M61 .22LR Shot are the straight thru one dia (.217"d) guns. These were made starting around the mid 50's.

There are a few fake Routledges around of course. Counter boring a straight thru bored rifle, or a Winchester counterbored gun to the correct .375d & depth results in a 'Routledge bore'.
The former will be way off in ser#/production. But the latter can be argued as being in the range. Be careful as always.
Check bbl address marking and other features for era of mfg.
No records available AFAIK on these.

Simmons offered the M61 22LR Shot w/a V/R installed.
It's pictured in their 1956 catalog,pg22
.."Model 61 Slide Action Hammerless Repeating Rifle
with New Simmons Deluxe Ventilated Rib
#G6116RSVR / .22Long Rifle Shot (Smooth Bore) / $97.85"

I'm guessing they'd fit one to a customers rifle as well.