The Triumph 3HW oil tank is similar to all oil tanks fitted to rigid frame Triumph models from 1937 onwards,there are a few differences with other rigid frame oil tanks.
below,the basic oil tank.

below,the oil tank is fitted with a brass screw on filler cap,same style as the petrol tank cap but smaller in dia.

below,details of the drain plug and rocker oil feed pipe take off on the oil return.

below,the oil filter removed on the feed side.

below,oil tank breather pipe differs from any other models with its screw type thread. above,breather pipe fits into hole on front frame under the saddle.
below,some of the oil tanks have a date stamp on the rear,inc post war tanks.

below,illustration from drivers hand book.

below,the oil tank also has two transfers,MINIMUM OIL LEVEL & IMPORTANT DRAIN OIL AND REFILL EVERY 2000 MILES.

Steve,
many thanks for sharing this, and sorry for replying so late.
I've never noticed the stamping on the rear of some of the tanks, I will go and check mine after seeing this.

Also, regarding the transfers, interesting that it shows the oil change mileage as 2000 miles, most Triumph transfers I've seen (including those on the VMCC transfers site) show 1500. I'm assuming on WD bikes these transfers would have been white? On restored bikes I'm certain I've seen gold, I'm unsure which is correct.

Regards,
Mark

Hi Mark,I think the oil tank transfers were gold in colour same as the petrol tanks,this is the only photo that I could find showing the oil tank transfers and their fixing locations,the drain and refill higher up than on the handbook drawing.