Greetings . . . welcome to our site!
Unfortunately, the guys from Flickr are trying prevent images from being cross-posted on sites like ours. Those URLs are only good when posted as links back to their site. This is because the URL they are providing needs to be processed on their site in order to retrieve the actual image. Once processed the pictures can be found in the page source (right click 'View Page Source' in Firefox - the link to the static pic is located at-or-about line 105 in the html page source).
Now to the pics. The crate is a nice find:
The part number on the crate is definitely for a fifty-four 261:
GM Parts Wiki I concur on the date code on the block - Feb of '57:

With a casting number of 3733950 and that date code you've got a '57 261 short block.
The serial number was likely transferred to this block:

in order to keep it consistent with paper-work on the vehicle. The fonts don't look factory to me - especially the FEA (the type face is to small). I have heard that some States used engine serial numbers for titles/bill-of-sale back in the day. Hence the transfer.
My only thought is that this was purchased as a replacement engine for a 1948 Chevy Truck ?
Indeed.
Based on the physical evidence at hand - we can say that there once was a '48 truck that was running this short block. For some reason it was pulled and replaced by the '54 that came of that crate. And then the '57 was put into the crate for safe keeping . . . supposition of course - but plausible given what is at hand.
Nice find. Now the sixty-four-thousand-dollar question is: why was the '57 pulled in favor of the '54?
regards,
stock49