I took my first Marauder rifle apart in 2012. The valve stem then, and everyone since, has had black plastic on it, not white . Maybe some of the first Marauder rifles used them ?.
If you purchased the rifle used, you may be able to find out the date of mfg. by examining the first numbers in the serial number on the reservoir. If indeed used, the prior owner may have changed out the valve.
As long as you don’t fill the rifle higher than the mfg. recommended pressure of 3000 psi, or run the rifle on CO2 ( or somehow get a defective part from the start ? ) the factory part should work for many hundreds or even thousands of shots.
FYI- I just ordered some the other day.
Part number and price were:
Part name: over molded valve stem
Part number: 1700-140
$1.72 each + shipping $4.00Crosman phone: 1-800-724-7486