I do not think every option could be ordered, only what Spiegel had listed in the catalogs which would only be what the rifles they had in inventory would accept. That's why all were drilled & tapped for scope blocks to take Mossberg's scopes so if they sold a scope, it could be mounted to any of the rifles they had.

Here' the 1936 catalog page, the artist drawings are not 100% correct, the front mount on the scope is too far forward and the bottom picture of the H is not very close at all! This catalog has Marlin scopes, starting in 1937 they only offered Mossberg scopes's of a special design just for them.
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I think now that there is better data on the Spiegel rifles from actual catalogs showing what Spiegel offered the old 'rumor' that the holes were for a Belding & Mull scope mount has pretty much been dispelled. The few pictures I've found of actual mounts look like like they require a different hole pattern.