Chalk marks on vintage pen take the form of small, white lettering on a pen barrel giving, usually, the make, model, nib size, and retail price of the pen:
This is an almost intact example of a chalk mark on a Parker 17. Part of the price is eroded but I think it was '49/-, forty nine shillings. This is pre-decimal U.K. currency, the changeover date from 'old to new' currency in U.K. was 15th February 1971. Pens produced, and chalk marked in the year or so prior to this date were dual marked, with prices shown in currencies, in this case it would have been 49/- . £2.45.
The marks are not actually chalk, but are applied in wax using a stencil. The marks were intended as an aid to the retailler as a quick and easy check that the pen being sold was actually the correct model at the correct price and with the reauired nib size. As the pen was sold the marks had served their purpose and were usually removed, they could be easily rubbed off without marking the pen.
The ease of removal is the significant feature of the marks to a collector. If the marks were not rubbed off at the time of purchase then, with normal use, they would erode quite rapidly. The presence of these marks is therefore a very strong indicator, though not proof, of sparing use.