Before I had my beloved Singer 503 I had a plastic Simplicity brand machine. It was a very fun beginner's machine. It was toward the end of the machine's life when my husband tore his work jeans. He had 5 minutes before he had to go to work. Plenty of time to mend it, I thought.
It should have been an easy fix. It was a tear along the back pocket. Just a bit of zigzagging and they'd be good as new. I threaded my machine, got the pants positioned, and .... nothing. The needle would not go through it. It was one layer of heavy denim. Poor Husband had to wear his good jeans to work.
That is not a problem you come across with your Singer Slant-o-matic. It can easily stitch through 6 layers of denim. I've seen it done.
I had an extra 401A I had up for sale on craigslist the other day. The lady who bought it told me bought her 401 in 1960 and it just gave up the ghost this year, 52 years later. When she came to my house she brought along a pair of her husband's work jeans with a tear so she could give the machine a test drive. It was amazing watching her mend the tear. She really knew what she was doing. It must have taken her and the 401 less than a minute to do the whole thing. The 401 zipped through that denim like it was nothing.
You really can't beat the Singer Slant-o-matic.