The 42 is Swan nomenclature for their standard lever ("self") fillers - "4" is for full length pen ("5" for vest pocket) and "2" is nib size / girth. In the early 20's, they marked the butt end of the barrel for nib size / girth only - towards the mid 20's, they started marking length as well.
I wouldn't expect to see anything underneath the 42 - in 26/27, they came out with a line of celluloids that indicated the color using a double digit number scheme. On the eternals, you would see "ETN" underneath. In the 30's, they dropped the color coding (on the American pens) and went back to barrel length / nib size numbering. So we can reasonably guess that this pen is 1921-25
The chasing on that pen is remarkably crisp, fabulous find.
In 1928, they started making the clip with the swan on it.
Oh dear! I should have worked that one out!
Yes 44ETN, 46ETN, 54ETN etc,
It was a question of being hidebound with all those SF2s I suppose, which does beg a question: the patent date in the imprint shows that this pen was definitely made after May 1918. I should like to know when the SF nomenclature was introduced. I had a SF2 with an engraved band dating the pen to January 1922. There is also the mystery of some pens being named 2SF as well.
All of these variations must (judging by the appearance of these pens) have occurred before about 1926. I have in front of me a Swan stamped as follows: (The Swan Image precedes the lettering as usual).
Swan Self filling Pen
Mabie Todd & Co Ltd London
Pat No 118118 May 1916 - Pat Pend.
Made in U.S.A.
Nib is a MT NY broad stub No 2 size
Next to it I have a a Swan of almost identical appearance with Swan SF2 stamped on the cap. The barrel reads
Mabie Todd & Co Ltd
Made in England.
Note the absence of patent information.
The cap is fitted with a Swan Clip - i.e. not a Pat 1915 one, but I should be surprised if this pen were later than 1924. Of course MT in London would fit a clip for a nominal sum so perhaps it was retro fitted. I cannot read the bottom of the nib and I don't want to pull it as the pen is writing nicely at present; as it states "14ct" one might assume that it is English - but perhaps made in NY for English applications before the English nib facility was established.
There seem to be endless possibilities for speculation with old Mabie Todds. What a shame we so not have a Mabie Todd (ALL) section on FPN.