Is this pen safe for writing on the back of photos? ("archival"?)

I think it is. RSVP pens are not gel based and don't smear. The writing is very thin. If they are important photos, I would test it out on something else first to make sure you're ok with the results.

Amazing. All that detailed information and no indication of whether or not it's retractable. Is it?

From the description: "This capped pen is designed for extended periods of use." It was easy to miss. :-) These are capped, but Pentel does make retractable pens. This one is retractable, comes in the same colors and widths, and has refills available. https://smile.amazon.com/Pentel-Super-Ballpoint-Medium-Violet/dp/B07D4Y7YXY/ref=sr_1_24?crid=IFNOX2JTO73E&keywords=pentel%2Bballpoint&qid=1670173944&sprefix=pentel%2Bballpoint%2Caps%2C117&sr=8-24&th=1 The item on this page is my favorite pen. I love writing in a fine line. I have some Pentel retractable pens, but they are medium width. I didn't know they made fine line retractable pens. Now, thanks to you, I have found the item I just suggested, so I am going to buy it - in several colors! I hope you enjoy these (or the other) pens as much as I do!

Manufacturer	Pentel
Brand	Pentel
Item Weight	4.8 ounces
Product Dimensions	0.65 x 6.1 x 0.65 inches
Item model number	BK91-A
Is Discontinued By Manufacturer	No
Color	Black
Closure	Snap
Grip Type	ergonomic
Pencil Lead Degree (Hardness)	HB
Material Type	Stainless Steel
Number of Items	12
Size	1 Count (Pack of 12)
Point Type	Medium
Line Size	1.0mm
Ink Color	Black
Tip Type	Metal
Manufacturer Part Number	BK91-A

RSVP Ballpoint

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