have a SAMMI throat. The SAMMI 45/70 throat is throatless, however, sometimes the case is shorter than the chamber, leaving a bit of a jump between the case mouth and the rifling. With a Marlin, it can go either way, with the case mouth right up against the rifling, or with a bit of a jump to the rifling. Also it will depend on your case trim length. Plus, if it is a used rifle, the previous owner may have firelapped it, which usually lengthens the throat.
If you really want to know, make an upset slug impression of the throat, and tell us what you find. If it's a new rifle, I'd lay money its a SAAMI throat.
This thread suggests a design for the Marlin 45/70. I've made quite a few of these and never had a single complaint about it. http://www.mountainmolds.com/phpBB3/viewtopic.php?f=7&t=74
A short bore riding section is a good idea in Marlin lever actions, and the suggested design has one.