So, I got this lens home. Have not found much info on the Internet, but found out that it was made around 1960, and it has just one lens group, a doublet. It is much lighter than the later f4.5 version and its rack and pinion focusing makes it easier and faster to focus than the f4.5 version.
http://www.pentaxforums.com/lensreviews/Takumar-500mm-F5.html
It came with a small vintage Linhoff tripod with a larger Linhoff ball head which I'll keep.
I'll put some film through it this week and post some scans.
Does anyone here have one of these?