Hi David-
My recollection is the tripmeter was incorporated on bikes destined for the US, though maybe not available in the earliest years of Earles bikes. (B&S also spec'd BMW to include things like passenger pegs/sidestand/benchseat/US bars for the US market)
As Kurt said, the 1,44 speedo is for sidecar gearing, specifically the 27:7 gears.
You may want to verify with a quick check what gears are currently in your machine, just to see if they match the installed speedo; this in case you do decide to go solo, as the 27:7 speedo and gearset are highly sought after.
BMW spec'd early Earles twins with 26:6 gears, and the /2 with the 27:7 set.
As for resetting the tripmeter- When not in use the knob rests on the rubber bush in the glass; when resetting the knob is lifted to engage the gears, engagement is felt at the top of extension. (refer to pics showing height difference between rest and engaged)
Turning the knob CW increases the numbers shown, turning CCW decreases the numbers; I turn in whichever direction is closest to zero out.
Just went out to the frozen barn to verify my memory of this, and found the knob to be stuck. (it was registering 536 mi. from last fall's rides, pretty close to yours...)
I used the tips of needle nose pliers under the knob, fulcruming off the bezel, to gently lift the knob; it was stubborn, and so proceeded slowly, not wanting to dent the bezel or crack the glass.
After full extension, it was pushed it in and out a number of times to loosen up till moving freely, before zeroing out.
As a lad the bike was often my only form of transport, and I was skint more often than not, and so ran for years and years with no functioning speedo.
These days I have more appreciation, and use, for speedo/odo/trip meter, and so it got rebuilt about ten years ago.
Dysfunction of the unit is mostly down to hardening of the lubricant, and the havoc that consequently raises between those moving and non-moving gears.
Probably most of the cost of the rebuild is in the disassembly/cleaning/lubing/reassembly, rather than in actual parts replacement, so perhaps no harm in waiting till other functions fail.
As Patrick said, the speedos can be quite accurate, with good tire and air pressure choice.
Best-
George