if you are sure i'll make a note. isn't the 257. # a block motor? along with the 397? it does look plastic so i'll take your word for it to pass on those.
You are correct, VM site lists Allegretti Co & Paramount as makers of m-257 and Paramount/Allegretti & Co (continued Delco grinder designs) as makers of m-397.
It also lists Paramount (now Poulan) as makers of m-257.
I have a plastic bodied 1/3 H.P. m-257.190470 variable-speed grinder, purchased new in 1987. I don't count it amongst the Block grinders.
It appears Sears began selling the plastic bodied m-257 again, leading to some confusion.
To me, if it has prefix 257 or 397
AND is a metal body it is the Block motor grinder by the Allegretti Co & Paramount supplier.
What makes the Block grinders made by these suppliers robust and desirable, is the quality: of the castings, bearings (same as used on some early Baldors I have opened), and gauge of field windings.
"General speaking, the commercial grinders are rated for industrial use and dual voltage.
The non-commercial grinders are usually single voltage 6" and some 7" for homeowner use." per Brian Kachadurian on OWWM.
"A point of clarification. Sears sold many "block motor" designed grinders from several manufacturers, but the block design must be considered secondary. Of primary concern is the manufacturer. The best engineered, most durable, and desirable "Block Motor" grinders were made by Paramount/Allegretti & Co. These grinders have the 397 or 257 serial number prefixes.
Overall, the "Block Motor" designs were also made by Rixon, Royal, and B&D -- just to name a few. These grinders do not have the higher quality field windings and castings that the Paramount/Allegretti units had." Source:
http://tinyurl.com/Block-grinder-manufacturers
I am not certain what the difference, if any, there are between commercial and industrial.
I hope this helps.