TomA, If you use the wood to metal line on the side of the guns to estimate position, all the pictures of Belding & Mull rear block show the front of the block just about even with that line, The Siegel hole pattern puts the front hole just about there so the hole for a screw in that position would break through the front of a Belding & Mull mount as pictured. The pictures of Belding & Mull mounts all seem to be of actual mounted scopes. It looks like your block would set much farther forward.
The long front mount for the Belding & Mull was used on a number of rifles, I had pictures of one that was not for a Savage but was the same design and it mounted with three screws, one in the center and one at close to each end. That long mount would not be very stable using the two narrow space holes on a Spiegle, especially the version that used just the front. I don't think Belding & Mull would change their standard hole patterns to one that was less secure for one retailer. I also never thought Spiegel would have chosen to have the guns they offered take a very expensive obscure scope, it would make not business sense at all for the type of retailer they were. It's been well documented that they offered reasonably priced Mossberg scopes on the 99's they sold.
Here are pictures from an old thread on the Spiegels -
These and others were in this thread, about the 8th post has more Belding & Mull pictures from a different catalog, this link should go right to that post -
threads/12073547/re-spiegel-infoHere's another from that thread -