Odds are that silent fast spiral side had a matrix number because it probably did get a commercial release of it's own as a servicing tool for shops that repaired record changers...As a diagnostic record release there were probably test tones on it's flip side. The fast spiral was a staple of changer/phonograph repair test discs. I have a set of test LPs from the 50's meant for evaluating/repairing HiFi systems and among the ~6 sides in the box set is a silent fast spiral similar to that. It is interesting to back a test pressing of a song with a diagnostic pressing.

You never know what you may find "junking"! In the mid 1970s I stopped in an antique store. This was near Indianapolis. I found 210 Gennett 10" in original sleeves. They appeared to be unplayed new old stock. I Bought them all even the religious, sentimental etc. 10c each, so $21, for them all. Mostly common but couple goodies worth $ even back then. The clerk there, an older woman, noticed the labels. She told me that her father and grandfather worked for Starr Piano which made Gennett's. They worked in the piano dept.She told me that the correct pronounciation was: Jennett. I was calling it Gahnett. I found a fair number of Gennett's as I lived in Indiana until 1997.

Very strange. I have some Victor test pressings with silent grooves but no matrix.

Interesting.I have a few sides like this that serve a definite purpose- - -on automatic drop sequence [not slide sequence] 78 rpm classical sets.The blank side with rapid feed groove kept the pick up arm under control & briskly caused the auto change mech to trip ,thus ending the play back cycle.

I've encountered a couple of records with silent grooves on one of the sides, and I've never been able to figure out why they chose to put that on there as opposed to a regular backplate.

Any cut groove should have a matrix number. One record I bought at a sale is a Freeman Lang one-sider titled "San Francisco All Americans", matrix A-835. It is over 16.5 inches in diameter and because of that I have not had a chance to play it. All attempts to find info on it have failed.