Is there a difference between the spr4 and the pr4?

The "PR" stands for Pressurized Refill and the PR4 is the black refill that fills most of the standard Fisher Space pens and other similar pens. If the code ends in a "B", then it's a bold, or ends in a "F" then it's a fine point. The numeric value after the PR indicates the color (1= blue 2=red, 3=green, 4=black, etc. I believe that the "SPR" simply means that the package includes the little plastic adapter piece that makes the refill Parker compatible. All my refills are the SPR version and work in either Fisher Space Pens (without the plastic adapter) or in my Parker Jotter pens (with the adapter). BTW - Fisher also makes refills with a label "C" like C1 or C4 which are for Cross pens.

Will this refill work in a Bullet space pen?

Greg K,This refill does work correctly in several Bullet Space Pens I purchased!

What is the tip size? I like a solid 1.0, would that be medium or bold?

I would have to say this is more of a medium than a bold. Most 1.0 pens that I have purchased like the Pilot G-2 or Uniball 207 Gel Impact are bold, which I prefer. The Fisher Space Pen refill is less so to me, but still good. What I REALLY like about the Fisher is how exceptionally smooth the ink flow is.

ASIN	B00168YNEO
Best Sellers Rank	#32,775 in Office Products ( See Top 100 in Office Products ) #5,978 in Pens & Refills
Brand	Fisher Space Pen
Brand Name	Fisher Space Pen
Country of Origin	USA
Customer Reviews	4.7 out of 5 stars 1,973 Reviews
Global Trade Identification Number	00747609111118, 07476091111186
Included Components	9004524
Ink Colour	Blue
Item Dimensions	8.9 x 0.5 x 0.5 Centimeters
Item Type	Refill
Item Weight	0.03 Pounds
Item dimensions L x W x H	8.9 x 0.5 x 0.5 Centimeters
Manufacturer	Fisher
Manufacturer Part Number	SPR1F
Model number	SPR1F
Number of Pieces	1
Point Type	Fine
Style	Military
Style Name	Military
UPC	097914378943 047609111119 747609111118
Unit Count	1.0 Count

Fisher Space Pen Point Refill, Blue Fine

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