The type they used back in the 1970's were not hydraulic fed. It had a square funnel like end with the blades spinning inside at the small part of the "funnel". No safety guards, no curtains, nothing. They were powered by V-8 auto type engines. Incredibly noisy even when not chipping.
Here is a picture of a similar "drum chipper" I used:
http://en.wikibooks.org/wiki/A_Wikimanual_of_Gardening/Woodchipper
To quote the article: The drum also serves as the feed mechanism, drawing the material through as it chips it. This caused it to be colloquially known as a "chuck-and-duck" chipper, because material would start moving through the chipper very quickly as soon as it made contact with the drum.
These chippers have many downsides. The drum-style chipper is not as safe as newer designs. If an operator becomes snagged on material being fed into the machine, injury or death is almost certain. These chippers are also very loud. "
In the 60’s I worked with a friend part time who worked with Asplundh. Used to feed branches into that thing and it sucked them in at unbelievable speed . Used to get whacked on the ears by branches as they wizzed by and that hurt . Couldn’t hear too well by end of day because of the noise level .