Thanks to all for the suggestions. My Markall pens arrived yesterday from Enco. So, I soon will be marking the filters I changed last weekend.
An intersting aside was the pens arrived in an 8" square box. I thought it was kind of large container for just a few pens. When I opened the box, the pens were in a resealable plastic bag, surrounded by packing 'peanuts', along with the invoice. Also in the box was a twice-folded document which I initially thought was an Enco catalog since it's not unusual for mail order places to include their current catalog with an order. Well, it wasn't a catalog.
It was two 10 page documents warning me about the various chemicals in the pens, proper disposal methods for the pens, etc. /forums/images/graemlins/blush.gif