Safe working loads of shackle blocks

I'm a fan of Garhauer Marine's blocks because they're pretty solid things that seem like they can take a pretty good beating, and relatively inexpensive. Practical Sailor has rated them well too, and they generally get good reviews on various forums and websites. People have good things to say about their quality and especially about their customer service.

However I am somewhat disappointed in the statements regarding the safe working load (SWL) for their shackle blocks. I bought several of their single stainless steel 60-13-sized shackle blocks and noticed that the shackles seemed a little smaller than what I expected. So I read the website a little more carefully: The Garhauer website states they use 1/4" shackles, and that the blocks have a SWF of 3500 lbs.

The 1/4" shackles made by Wichard have a SWL of about 1800 lbs. So I don't understand how the Garhauer shackle blocks could have a SWL of almost double a high-quality 1/4" shackle.

I sent an email to Garhauer to ask; they replied that the website was incorrect and that these were 5/16th shackles. So I measured the shackles on the 4 blocks I had purchased:

The pin diameter was 19/64th, which is  I guess within tolerance of 5/16ths.

However the shackle body was not that size. The shackle body is slightly rectangular, not round. The "inside" diameter of the saddle was 15/64 and the "outside" diameter was 9/32 -- these are both pretty much 1/4".

I wrote back to Garhauer and they replied that shackles are measured by their clevis pin diameter. They are not actually,

Shackles are sized according to the diameter of the bow section rather than the pin size.

(Specifically by the diameter of the "saddle" in the bow-section.)

But in any case I replaced the shackles on my blocks with "true" 5/16th shackles. (I measured it to be sure)

It is important to note that the strain on shackles have to be centered on the pin, by using washers to act as spacers. If the strain is placed on one end, you could end up losing 70% of the SWL of the shackle.