I have never owned a model M54 but I do have two Schultz & Larsens and would venture to say they are my favorite bolt action. The 1st one I got is a 65DL in 308 Norma Magnum and the 2nd, recently acquired, is a 68DL in 7x61 S&H. Eventually I hope to get one more Schultz, in 358 Norma Magnum.
I believe it was in the late 30's and early 40's that a significant amount of experimenting with the 7mm caliber by a number of people took place. This resulted in a co-operative effort between Norma of Sweden and Schultz & Larsen of Denmark in bringing about and marketing the 7x61 Sharpe & Hart cartridge about 1953. Any popularity the cartridge had was over shadowed roughly ten years later when Remington brought out the 7mm Remington Magnum. About 1968, to counteract the interest decline in the 7x61 S&H, Norma made change improvments to the metallurgy. This, through thinning the walls and altering the case interior profile, allowed them to actually increase the case capacity and make it a closer competitor to the 7mm Remington Magnum. This later production brass it designated the 7x61 S&H Super.
I have done some reloading and shooting with some of the more traditional bullets available but I'm now in the process of developing loads using Barnes TTSX bullets. I'm hoping to be successful in an Antelope draw in AB within the next couple of years and plan on using the 7x61 S&H.
If you were looking to buy a 7mm, there are more appropiate choices from some of the more readily availvble cartridges of today. With the 7x61 S&H and if you reload, components like brass can be a little hard to come by but once you get it and dies, it's just like reloading any other caliber.